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:paddingBottom="@dimen/activity_vertical_margin"
                android:paddingLeft="@dimen/activity_horizontal_margin"
                android:paddingRight="@dimen/activity_horizontal_margin"
                android:paddingTop="@dimen/activity_vertical_margin"
                tools:context=".MainActivity">

    <EditText
        android:id="@+id/result_id"
        android:layout_width="match_parent"
        android:layout_height="70dp" />

    <Button
        android:id="@+id/Btn7_id"
        android:layout_width="70dp"
        android:layout_height="60dp"
        android:layout_below="@id/result_id"
        android:onClick="btn7Clicked"
        android:text="7" />

    <Button
        android:id="@+id/Btn8_id"
        android:layout_width="70dp"
        android:layout_height="60dp"
        android:layout_below="@id/result_id"
        android:layout_toRightOf="@id/Btn7_id"
        android:onClick="btn8Clicked"
        android:text="8" />

    <Button
        android:id="@+id/Btn9_id"
        android:layout_width="70dp"
        android:layout_height="60dp"
        android:layout_below="@id/result_id"
        android:layout_toRightOf="@id/Btn8_id"
        android:onClick="btn9Clicked"
        android:text="9" />

    <Button
        android:id="@+id/Btnclear_id"
        android:layout_width="90dp"
        android:layout_height="60dp"
        android:layout_below="@id/result_id"
        android:layout_toRightOf="@id/Btn9_id"
        android:onClick="btnclearClicked"
        android:text="clear" />

    <Button
        android:id="@+id/Btn4_id"
        android:layout_width="70dp"
        android:layout_height="60dp"
        android:layout_below="@id/Btn7_id"
        android:onClick="btn4Clicked"
        android:text="4" />

    <Button
        android:id="@+id/Btn5_id"
        android:layout_width="70dp"
        android:layout_height="60dp"
        android:layout_below="@id/Btn8_id"
        android:layout_toRightOf="@id/Btn4_id"
        android:onClick="btn5Clicked"
        android:text="5" />

    <Button
        android:id="@+id/Btn6_id"
        android:layout_width="70dp"
        android:layout_height="60dp"
        android:layout_below="@id/Btn9_id"
        android:layout_toRightOf="@id/Btn5_id"
        android:onClick="btn6Clicked"
        android:text="6" />

    <Button
        android:id="@+id/Btnplus_id"
        android:layout_width="90dp"
        android:layout_height="60dp"
        android:layout_below="@id/Btnclear_id"
        android:layout_toRightOf="@id/Btn6_id"
        android:onClick="btnplusClicked"
        android:text="+" />

    <Button
        android:id="@+id/Btn1_id"
        android:layout_width="70dp"
        android:layout_height="60dp"
        android:layout_below="@id/Btn4_id"
        android:onClick="btn1Clicked"
        android:text="1" />

    <Button
        android:id="@+id/Btn2_id"
        android:layout_width="70dp"
        android:layout_height="60dp"
        android:layout_below="@id/Btn5_id"
        android:layout_toRightOf="@id/Btn1_id"
        android:onClick="btn2Clicked"
        android:text="2" />

    <Button
        android:id="@+id/Btn3_id"
        android:layout_width="70dp"
        android:layout_height="60dp"
        android:layout_below="@id/Btn6_id"
        android:layout_toRightOf="@id/Btn2_id"
        android:onClick="btn3Clicked"
        android:text="3" />

    <Button
        android:id="@+id/Btnminus_id"
        android:layout_width="90dp"
        android:layout_height="60dp"
        android:layout_below="@id/Btnplus_id"
        android:layout_toRightOf="@id/Btn3_id"
        android:onClick="btnminusClicked"
        android:text="-" />

    <Button
        android:id="@+id/Btnequal_id"
        android:layout_width="110dp"
        android:layout_height="60dp"
        android:layout_below="@id/Btn1_id"
        android:onClick="btnequalClicked"
        android:text="=" />

    <Button
        android:id="@+id/Btndivide_id"
        android:layout_width="90dp"
        android:layout_height="60dp"
        android:layout_below="@id/Btn1_id"
        android:layout_toRightOf="@id/Btnequal_id"
        android:onClick="btndivideClicked"
        android:text="/" />

    <Button
        android:id="@+id/Btnmulti_id"
        android:layout_width="100dp"
        android:layout_height="60dp"
        android:layout_below="@id/Btnminus_id"
        android:layout_toRightOf="@id/Btndivide_id"
        android:onClick="btnmultiClicked"
        android:text="*" />

</RelativeLayout>