diff --git a/app/src/main/java/io/n0x/android/java_mot_testing/FractionCalculator.java b/app/src/main/java/io/n0x/android/java_mot_testing/FractionCalculator.java
index b0a904a..5970546 100644
--- a/app/src/main/java/io/n0x/android/java_mot_testing/FractionCalculator.java
+++ b/app/src/main/java/io/n0x/android/java_mot_testing/FractionCalculator.java
@@ -8,79 +8,159 @@ import androidx.appcompat.app.AppCompatActivity;
 import androidx.databinding.BaseObservable;
 import androidx.databinding.Bindable;
 
-public class FractionCalculator extends BaseObservable{
+import java.security.InvalidParameterException;
 
+public class FractionCalculator extends BaseObservable{
+    // Debugging stuff
     private static final String TAG = FractionCalculator.class.getName();
 
-    private String denomLeft;
-    private String enumLeft;
-    private String denomRight;
-    private String enumRight;
-    private String denomResult;
-    private String enumResult;
+    public enum ARTIH {ADD, SUBTRACT, MULTIPLICATE, DIVIDE};
+    private ARTIH arithMethod;
+
+    private String centerSign;
+
+    private int denomLeft;
+    private int enumLeft;
+    private int denomRight;
+    private int enumRight;
+    private int denomResult;
+    private int enumResult;
 
     public FractionCalculator(){
         Log.d(TAG, "new FractionCalculator created");
     }
 
-    /// #### GETTER AND SETTER ####
+    // Calculate the result
+    // https://de.wikibooks.org/wiki/C%2B%2B-Programmierung/_Br%C3%BCche
+    public void calculateResult(){
+        if(denomLeft == 0 || denomRight == 0){
+            Log.e(TAG, "Denominator cannot be 0");
+        } else {
+            switch (arithMethod) {
+                case ADD:
+                    denomResult = getLCM(denomLeft, denomRight);
+                    enumResult = enumLeft * (denomResult / denomLeft) + enumRight * (denomResult / denomRight);
+                    break;
+                case SUBTRACT:
+                    denomResult = getLCM(denomLeft, denomRight);
+                    enumResult = enumLeft * (denomResult / denomLeft) - enumRight * (denomResult / denomRight);
+                    break;
+                case MULTIPLICATE:
+                    denomResult = denomLeft * denomRight;
+                    enumResult = enumLeft * enumRight;
+                    break;
+                case DIVIDE:
+                    enumResult = enumLeft * denomRight;
+                    denomResult = denomLeft * enumRight;
+                    break;
+            }
+            // update the result variables
+            notifyPropertyChanged(BR.denomResult);
+            notifyPropertyChanged(BR.enumResult);
+        }
+    }
+
+    // ####### HELPER METHODS #######
+    // calculate greatest common divisor (Euclidean algorithm)
+    private int getGCD(int a, int b){
+        if(b==0){
+            return a;
+        } else {
+            return getGCD(b,a % b);
+        }
+    }
+
+    // calculate least common multiple
+    private int getLCM(int a, int b){
+        return (a*b)/getGCD(a,b);
+    }
+    // ####### END HELPER METHODS #######
+
+    /// ####### GETTER AND SETTER #######
+    public void setArithMethod(ARTIH arithMethod) {
+        this.arithMethod = arithMethod;
+        switch(arithMethod){
+            case ADD:
+                centerSign = "+";
+                break;
+            case SUBTRACT:
+                centerSign = "-";
+                break;
+            case MULTIPLICATE:
+                centerSign = "*";
+                break;
+            case DIVIDE:
+                centerSign = "/";
+                break;
+        }
+        // Update the sign in the middle of the screen
+        notifyPropertyChanged(BR.centerSign);
+    }
+
     @Bindable
-    public String getDenomLeft() {
+    public int getDenomLeft() {
         return denomLeft;
     }
 
-    public void setDenomLeft(String denomLeft) {
+    public void setDenomLeft(int denomLeft) {
         this.denomLeft = denomLeft;
         notifyPropertyChanged(BR.denomLeft);
     }
 
     @Bindable
-    public String getEnumLeft() {
+    public int getEnumLeft() {
         return enumLeft;
     }
 
-    public void setEnumLeft(String enumLeft) {
+    public void setEnumLeft(int enumLeft) {
         this.enumLeft = enumLeft;
         notifyPropertyChanged(BR.enumLeft);
     }
 
     @Bindable
-    public String getDenomRight() {
+    public int getDenomRight() {
         return denomRight;
     }
 
-    public void setDenomRight(String denomRight) {
+    public void setDenomRight(int denomRight) {
         this.denomRight = denomRight;
         notifyPropertyChanged(BR.denomRight);
     }
 
     @Bindable
-    public String getEnumRight() {
+    public int getEnumRight() {
         return enumRight;
     }
 
-    public void setEnumRight(String enumRight) {
+    public void setEnumRight(int enumRight) {
         this.enumRight = enumRight;
         notifyPropertyChanged(BR.enumRight);
     }
 
     @Bindable
-    public String getDenomResult() {
+    public int getDenomResult() {
         return denomResult;
     }
 
-    public void setDenomResult(String denomResult) {
+    public void setDenomResult(int denomResult) {
         this.denomResult = denomResult;
         notifyPropertyChanged(BR.denomResult);
     }
 
     @Bindable
-    public String getEnumResult() {
+    public int getEnumResult() {
         return enumResult;
     }
 
-    public void setEnumResult(String enumResult) {
+    public void setEnumResult(int enumResult) {
         this.enumResult = enumResult;
         notifyPropertyChanged(BR.enumResult);
     }
+
+    @Bindable
+    public String getCenterSign() {
+        return centerSign;
+    }
+
+    /// ####### END GETTER AND SETTER #######
 }
diff --git a/app/src/main/java/io/n0x/android/java_mot_testing/MainActivity.java b/app/src/main/java/io/n0x/android/java_mot_testing/MainActivity.java
index 865efe6..4e6e6f9 100644
--- a/app/src/main/java/io/n0x/android/java_mot_testing/MainActivity.java
+++ b/app/src/main/java/io/n0x/android/java_mot_testing/MainActivity.java
@@ -1,5 +1,6 @@
 package io.n0x.android.java_mot_testing;
 
+import android.annotation.SuppressLint;
 import android.os.Bundle;
 import android.util.Log;
 import android.view.View;
@@ -16,6 +17,8 @@ public class MainActivity extends AppCompatActivity implements View.OnClickListe
     // Local calculator instance
     private FractionCalculator calc;
 
+    private int test;
+
     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
@@ -23,8 +26,8 @@ public class MainActivity extends AppCompatActivity implements View.OnClickListe
         calc = new FractionCalculator();
 
         // set a value for testing
-        calc.setEnumResult("0");
-        calc.setDenomResult("0");
+        calc.setEnumResult(0);
+        calc.setDenomResult(0);
 
         // Bind instance of FractionCalculator to view
         Ex9DataBindingBinding binding = DataBindingUtil.setContentView(this, R.layout.ex9_data_binding);
@@ -32,21 +35,34 @@ public class MainActivity extends AppCompatActivity implements View.OnClickListe
 
         // Button Listeners
         findViewById(R.id.button_add).setOnClickListener(this);
+        findViewById(R.id.button_sub).setOnClickListener(this);
+        findViewById(R.id.button_mult).setOnClickListener(this);
+        findViewById(R.id.button_div).setOnClickListener(this);
+        findViewById(R.id.button_calc).setOnClickListener(this);
 
         // Log create action
         Log.d(TAG, "MainActivity started");
     }
 
+    @SuppressLint("NonConstantResourceId")
     @Override
     public void onClick(View v) {
-        if (v.getId() == R.id.button_add) {
-
-            // works (without using bindings)
-            TextView textView = findViewById(R.id.txt_enum_result);
-            textView.setText("1234");
-
-            // work with DataBinding
-            calc.setDenomLeft("123123123");
+        switch(v.getId()){
+            case R.id.button_add:
+                calc.setArithMethod(FractionCalculator.ARTIH.ADD);
+                break;
+            case R.id.button_sub:
+                calc.setArithMethod(FractionCalculator.ARTIH.SUBTRACT);
+                break;
+            case R.id.button_mult:
+                calc.setArithMethod(FractionCalculator.ARTIH.MULTIPLICATE);
+                break;
+            case R.id.button_div:
+                calc.setArithMethod(FractionCalculator.ARTIH.DIVIDE);
+                break;
+            case R.id.button_calc:
+                calc.calculateResult();
+                break;
         }
     }
 }
\ No newline at end of file
diff --git a/app/src/main/res/layout/ex9_data_binding.xml b/app/src/main/res/layout/ex9_data_binding.xml
index 6f2a305..0419486 100644
--- a/app/src/main/res/layout/ex9_data_binding.xml
+++ b/app/src/main/res/layout/ex9_data_binding.xml
@@ -15,131 +15,148 @@
 
         <Button
             android:id="@+id/button_add"
-            android:layout_width="wrap_content"
+            android:layout_width="0dp"
             android:layout_height="wrap_content"
+            android:layout_marginStart="8dp"
+            android:layout_marginTop="152dp"
+            android:layout_marginEnd="8dp"
             android:text="+"
-            app:layout_constraintEnd_toEndOf="parent"
+            app:layout_constraintEnd_toStartOf="@+id/button_sub"
             app:layout_constraintStart_toStartOf="parent"
             app:layout_constraintTop_toTopOf="parent" />
 
         <Button
             android:id="@+id/button_sub"
-            android:layout_width="wrap_content"
+            android:layout_width="0dp"
             android:layout_height="wrap_content"
+            android:layout_marginTop="152dp"
+            android:layout_marginEnd="8dp"
             android:text="-"
-            app:layout_constraintEnd_toEndOf="parent"
-            app:layout_constraintStart_toStartOf="parent"
-            app:layout_constraintTop_toBottomOf="@+id/button_add" />
+            app:layout_constraintEnd_toStartOf="@+id/button_mult"
+            app:layout_constraintTop_toTopOf="parent" />
 
         <Button
             android:id="@+id/button_mult"
-            android:layout_width="wrap_content"
+            android:layout_width="0dp"
             android:layout_height="wrap_content"
+            android:layout_marginTop="152dp"
+            android:layout_marginEnd="8dp"
             android:text="*"
-            app:layout_constraintEnd_toEndOf="parent"
-            app:layout_constraintStart_toStartOf="parent"
-            app:layout_constraintTop_toBottomOf="@+id/button_sub" />
+            app:layout_constraintEnd_toStartOf="@+id/button_div"
+            app:layout_constraintTop_toTopOf="parent" />
 
         <Button
             android:id="@+id/button_div"
-            android:layout_width="wrap_content"
+            android:layout_width="0dp"
             android:layout_height="wrap_content"
+            android:layout_marginTop="152dp"
+            android:layout_marginEnd="8dp"
             android:text="/"
             app:layout_constraintEnd_toEndOf="parent"
-            app:layout_constraintHorizontal_bias="0.501"
-            app:layout_constraintStart_toStartOf="parent"
-            app:layout_constraintTop_toBottomOf="@+id/button_mult" />
-
-        <TextView
-            android:id="@+id/txt_equals"
-            android:layout_width="wrap_content"
-            android:layout_height="wrap_content"
-            android:layout_marginTop="60dp"
-            android:text="="
-            android:textSize="34sp"
-            app:layout_constraintEnd_toEndOf="parent"
-            app:layout_constraintHorizontal_bias="0.181"
-            app:layout_constraintStart_toStartOf="parent"
-            app:layout_constraintTop_toBottomOf="@+id/button_div" />
+            app:layout_constraintTop_toTopOf="parent" />
 
         <TextView
             android:id="@+id/txt_enum_result"
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
-            android:layout_marginTop="64dp"
-            android:text="@{calc.enumResult}"
+            android:layout_marginTop="16dp"
+            android:text="@{String.valueOf(calc.enumResult)}"
             android:textAlignment="center"
             android:textSize="34sp"
-            app:layout_constraintBottom_toTopOf="@+id/txt_denom_result"
             app:layout_constraintEnd_toEndOf="parent"
             app:layout_constraintStart_toStartOf="parent"
-            app:layout_constraintTop_toBottomOf="@+id/button_div" />
+            app:layout_constraintTop_toBottomOf="@+id/button_mult" />
 
         <TextView
             android:id="@+id/txt_denom_result"
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
-            android:layout_marginTop="3dp"
-            android:layout_marginBottom="412dp"
-            android:text="@{calc.denomResult}"
+            android:text="@{String.valueOf(calc.denomResult)}"
             android:textAlignment="center"
             android:textSize="34sp"
-            app:layout_constraintBottom_toBottomOf="parent"
             app:layout_constraintEnd_toEndOf="parent"
+            app:layout_constraintHorizontal_bias="0.501"
             app:layout_constraintStart_toStartOf="parent"
             app:layout_constraintTop_toBottomOf="@+id/txt_enum_result" />
 
         <EditText
-            android:text="@{calc.enumRight}"
             android:id="@+id/enumRight"
             android:layout_width="140dp"
             android:layout_height="50dp"
-            android:layout_marginTop="32dp"
+            android:layout_marginTop="16dp"
+            android:layout_marginEnd="16dp"
             android:ems="10"
+            android:gravity="center_horizontal"
             android:hint="Zähler"
             android:inputType="numberSigned"
+            android:text="@={`` + calc.enumRight}"
             app:layout_constraintEnd_toEndOf="parent"
-            app:layout_constraintStart_toEndOf="@+id/button_add"
             app:layout_constraintTop_toTopOf="parent" />
 
         <EditText
-            android:text="@{calc.enumRight}"
             android:id="@+id/enumLeft"
             android:layout_width="140dp"
             android:layout_height="50dp"
-            android:layout_marginTop="32dp"
+            android:layout_marginStart="16dp"
+            android:layout_marginTop="16dp"
             android:ems="10"
+            android:gravity="center_horizontal"
             android:hint="Zähler"
             android:inputType="numberSigned"
-            app:layout_constraintEnd_toStartOf="@+id/button_add"
+            android:text="@={`` + calc.enumLeft}"
             app:layout_constraintStart_toStartOf="parent"
             app:layout_constraintTop_toTopOf="parent" />
 
         <EditText
-            android:text="@{calc.denomLeft}"
             android:id="@+id/denomLeft"
             android:layout_width="140dp"
             android:layout_height="50dp"
-            android:layout_marginTop="32dp"
+            android:layout_marginStart="16dp"
+            android:layout_marginTop="16dp"
             android:ems="10"
+            android:gravity="center_horizontal"
             android:hint="Nenner"
             android:inputType="number"
-            app:layout_constraintEnd_toStartOf="@+id/button_mult"
+            android:text="@={`` + calc.denomLeft}"
             app:layout_constraintStart_toStartOf="parent"
             app:layout_constraintTop_toBottomOf="@+id/enumLeft" />
 
         <EditText
-            android:text="@{calc.denomRight}"
             android:id="@+id/denomRight"
             android:layout_width="140dp"
             android:layout_height="50dp"
-            android:layout_marginTop="32dp"
+            android:layout_marginTop="16dp"
+            android:layout_marginEnd="16dp"
             android:ems="10"
+            android:gravity="center_horizontal"
             android:hint="Nenner"
             android:inputType="number"
+            android:text="@={`` + calc.denomRight}"
             app:layout_constraintEnd_toEndOf="parent"
-            app:layout_constraintStart_toEndOf="@+id/button_mult"
             app:layout_constraintTop_toBottomOf="@+id/enumRight" />
 
+        <Button
+            android:id="@+id/button_calc"
+            android:layout_width="wrap_content"
+            android:layout_height="wrap_content"
+            android:layout_marginStart="8dp"
+            android:layout_marginTop="236dp"
+            android:text="="
+            app:layout_constraintStart_toStartOf="parent"
+            app:layout_constraintTop_toTopOf="parent" />
+
+        <TextView
+            android:id="@+id/centerSign"
+            android:layout_width="wrap_content"
+            android:layout_height="wrap_content"
+            android:gravity="center"
+            android:text="@{calc.centerSign}"
+            android:textAlignment="center"
+            android:textSize="60sp"
+            app:layout_constraintBottom_toTopOf="@+id/button_mult"
+            app:layout_constraintEnd_toStartOf="@+id/enumRight"
+            app:layout_constraintStart_toEndOf="@+id/enumLeft"
+            app:layout_constraintTop_toTopOf="parent" />
+
     </androidx.constraintlayout.widget.ConstraintLayout>
 </layout>
\ No newline at end of file