If you were unable to hear everything i said in the video above, below is the source code of the calculator application that was built.

Place this code in the layout of the application
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" android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin" tools:context=".MainActivity">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:text="My Basic Calculator"
        android:id="@+id/textView"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true"
        android:textColor="#008888" />

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceMedium"
        android:text="First Number"
        android:id="@+id/textView2"
        android:layout_below="@+id/textView"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:layout_marginTop="30dp" />

    <EditText
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:inputType="number"
        android:ems="10"
        android:id="@+id/etFirstNumber"
        android:layout_below="@+id/textView2"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true" />

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceMedium"
        android:text="Second Number"
        android:id="@+id/textView3"
        android:layout_below="@+id/etFirstNumber"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true" />

    <EditText
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:inputType="number"
        android:ems="10"
        android:id="@+id/etSecondNumber"
        android:layout_below="@+id/textView3"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true" />

    <LinearLayout
        android:orientation="horizontal"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_below="@+id/etSecondNumber"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true">

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="+"
            android:id="@+id/btnAdd"
            android:background="#51ff00" />

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="-"
            android:id="@+id/btnSubtract"
            android:background="#ff0000" />

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="/"
            android:id="@+id/btnDivide"
            android:background="#ffb300" />

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="*"
            android:id="@+id/btnMultiply"
            android:background="#00ffe6" />
    </LinearLayout>

    <LinearLayout
        android:orientation="horizontal"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_centerVertical="true"
        android:layout_alignParentRight="true"
        android:layout_alignParentEnd="true">

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:textAppearance="?android:attr/textAppearanceLarge"
            android:id="@+id/tvResult"
            android:layout_centerVertical="true"
            android:layout_alignParentLeft="true"
            android:layout_alignParentStart="true"
            android:textColor="#0000FF"
            android:layout_marginTop="40dp" />
    </LinearLayout>
</RelativeLayout>

Next, place this code in the MainActivity of the application
MainActivity.java

package com.basiccalc.mybasiccalculator;

import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;


public class MainActivity extends ActionBarActivity implements View.OnClickListener {
    private Button btnAdd,btnsub,btndivide,btnmul;
    private TextView tvresult;

    private EditText etfirst,etsecond;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        init();
    }

    private void init() {
        btnAdd =(Button)findViewById(R.id.btnAdd);
        btnsub = (Button)findViewById(R.id.btnSubtract);
        btndivide = (Button)findViewById(R.id.btnDivide);
        btnmul = (Button)findViewById(R.id.btnMultiply);
        etfirst = (EditText)findViewById(R.id.etFirstNumber);
        etsecond =(EditText)findViewById(R.id.etSecondNumber);
        tvresult = (TextView)findViewById(R.id.tvResult);

        btnAdd.setOnClickListener(this);
        btnsub.setOnClickListener(this);
        btndivide.setOnClickListener(this);
        btnmul.setOnClickListener(this);
    }

    @Override
    public void onClick(View view) {
        String num1 = etfirst.getText().toString();
        String num2 = etsecond.getText().toString();
        switch(view.getId()){
            case R.id.btnAdd:
                int addition = Integer.parseInt(num1) + Integer.parseInt(num2);
                tvresult.setText(String.valueOf(addition));
                break;
            case R.id.btnSubtract:
                int subtraction = Integer.parseInt(num1) - Integer.parseInt(num2);
                tvresult.setText(String.valueOf(subtraction));
                break;
            case R.id.btnDivide:
                try{
                    int division = Integer.parseInt(num1) / Integer.parseInt(num2);
                    tvresult.setText(String.valueOf(division));
                }catch(Exception e){
                    tvresult.setText("Cannot DIVIDE!");
                }
                break;
            case R.id.btnMultiply:
                int multiply = Integer.parseInt(num1) * Integer.parseInt(num2);
                tvresult.setText(String.valueOf(multiply));
                break;
        }
    }
}