[Java for Android App Development] Rypac:
I've fixed it up a little and it runs perfectly on my phone.

MainActivity.java

package com.ptx.calcplus;

import android.app.Activity;
import android.os.Bundle;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;

public class MainActivity extends Activity {
private EditText txtN1;
private EditText txtN2;
private EditText txtAn;
private Button Add, Sub, Div, Mul;
double a = 0, b = 0, c = 0;

[MENTION=439709]override[/MENTION]
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Add = (Button) findViewById(R.id.btnAdd);
Sub = (Button) findViewById(R.id.btnSub);
Div = (Button) findViewById(R.id.btnDiv);
Mul = (Button) findViewById(R.id.btnMul);
txtN1 = (EditText) findViewById(R.id.txtNum1);
txtN2 = (EditText) findViewById(R.id.txtNum2);
txtAn = (EditText) findViewById(R.id.txtAns);

Add.setOnClickListener(new View.OnClickListener() {
[MENTION=439709]override[/MENTION]
public void onClick(View arg0) {
a = Double.parseDouble(txtN1.getText().toString());
b = Double.parseDouble(txtN2.getText().toString());
c = a + b;
txtAn.setText(""+c);
}
});

Sub.setOnClickListener(new View.OnClickListener() {
[MENTION=439709]override[/MENTION]
public void onClick(View arg0) {
a = Double.parseDouble(txtN1.getText().toString());
b = Double.parseDouble(txtN2.getText().toString());
txtAn.setText(""+c);
}
});

Mul.setOnClickListener(new View.OnClickListener() {
[MENTION=439709]override[/MENTION]
public void onClick(View arg0) {
a = Double.parseDouble(txtN1.getText().toString());
b = Double.parseDouble(txtN2.getText().toString());
c = a * b;
txtAn.setText(""+c);
}
});

Div.setOnClickListener(new View.OnClickListener() {
[MENTION=439709]override[/MENTION]
public void onClick(View arg0) {
a = Double.parseDouble(txtN1.getText().toString());
b = Double.parseDouble(txtN2.getText().toString());
c = a / b;
txtAn.setText(""+c);
}
});
}

[MENTION=439709]override[/MENTION]
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.activity_main, menu);
return true;
}

}

activity_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity" >

<Button
android:id="@+id/btnMul"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignBaseline="@+id/btnSub"
android:layout_alignBottom="@+id/btnSub"
android:layout_alignParentLeft="true"
android:text="@string/btnm"
android:onClick="mul" />

<Button
android:id="@+id/btnSub"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
android:layout_marginBottom="127dp"
android:layout_toRightOf="@+id/btnMul"
android:text="@string/btns"
android:onClick="sub" />

<Button
android:id="@+id/btnAdd"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignBaseline="@+id/btnSub"
android:layout_alignBottom="@+id/btnSub"
android:layout_toRightOf="@+id/btnSub"
android:text="@string/btnp"
android:onClick="add" />

<Button
android:id="@+id/btnDiv"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignBaseline="@+id/btnAdd"
android:layout_alignBottom="@+id/btnAdd"
android:layout_toRightOf="@+id/btnAdd"
android:text="@string/btnd"
android:onClick="div"/>

<EditText
android:id="@+id/txtNum2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_above="@+id/btnMul"
android:layout_alignLeft="@+id/btnMul"
android:layout_alignRight="@+id/btnDiv"
android:layout_marginBottom="90dp"
android:ems="10"
android:hint="@string/txt2"
android:inputType="numberDecimal" />

<EditText
android:id="@+id/txtNum1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_above="@+id/txtNum2"
android:layout_alignLeft="@+id/txtNum2"
android:layout_alignRight="@+id/txtNum2"
android:layout_marginBottom="39dp"
android:ems="10"
android:hint="@string/txt1"
android:inputType="numberDecimal" >

<requestFocus />
</EditText>

<EditText
android:id="@+id/txtAns"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/txtNum2"
android:layout_alignRight="@+id/btnDiv"
android:layout_below="@+id/btnMul"
android:layout_marginTop="40dp"
android:ems="10"
android:enabled="false"
android:hint="@string/txta"
android:linksClickable="false" />

</RelativeLayout>

I haven't touched the other files. Hope you get it working.