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:background="@drawable/background"    >

    <EditText
        android:layout_width="match_parent"
        android:layout_height="40dip"
        android:layout_marginLeft="20dip"
        android:layout_marginRight="20dip"
        android:scrollHorizontally="true"
        android:inputType="number"
        android:focusable="false"
        android:background="#80000000"
        android:ems="10"
        android:id="@+id/number_edit"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="127dp" />

    <GridLayout
        android:layout_centerInParent="true"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/number_edit"
        android:layout_centerHorizontal="true"
        android:columnCount="3"
        android:orientation="horizontal">

        <Button android:text="1"

            android:layout_rowWeight="0.33"
            android:id="@+id/text_1"
            android:onClick="input"/>

        <Button android:text="2"
            android:layout_rowWeight="0.33"
            android:id="@+id/text_2"
            android:onClick="input"/>

        <Button android:text="3"
            android:id="@+id/text_3"
            android:layout_rowWeight="0.33"
            android:onClick="input"/>

        <Button android:text="4"
            android:id="@+id/text_4"
            android:layout_rowWeight="0.33"
            android:onClick="input"/>

        <Button android:text="5"
            android:id="@+id/text_5"
            android:layout_rowWeight="0.33"
            android:onClick="input"/>

        <Button android:text="6"
            android:id="@+id/text_6"
            android:layout_rowWeight="0.33"
            android:onClick="input"/>
        <Button android:text="7"
            android:id="@+id/text_7"
            android:layout_rowWeight="0.33"
            android:onClick="input"/>
        <Button android:text="8"
            android:id="@+id/text_8"
            android:layout_rowWeight="0.33"
            android:onClick="input"/>
        <Button android:text="9"
            android:id="@+id/text_9"
            android:layout_rowWeight="0.33"
            android:onClick="input"/>
        <Button android:text="*"
            android:id="@+id/text_cross"
            android:layout_rowWeight="0.33"
            android:onClick="input"/>
        <Button android:text="0"
            android:id="@+id/text_0"
            android:layout_rowWeight="0.33"
            android:onClick="input"/>
        <Button android:text="X"
            android:layout_rowWeight="0.33"
            android:id="@+id/text_backspace"
            android:onClick="input"/>
        <Button android:text="CALL"
            android:layout_rowWeight="1"
            android:layout_gravity="fill"
            android:layout_columnSpan="3"
            android:id="@+id/text_CALL"
            android:onClick="input"/>
    </GridLayout>

</RelativeLayout>