buttons.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/calculator_buttons"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <!--1-->
    <Button
        android:id="@+id/button1"
        android:text="@string/BUTTON_1"
        style="@style/Number"/>

    <!--2-->
    <Button
        android:id="@+id/button2"
        android:layout_toRightOf="@id/button1"
        android:text="@string/BUTTON_2"
        style="@style/Number"/>

    <!--3-->
    <Button
        android:id="@+id/button3"
        android:layout_toRightOf="@id/button2"
        android:text="@string/BUTTON_3"
        style="@style/Number"/>

    <!--4-->
    <Button
        android:id="@+id/button4"
        android:layout_below="@id/button1"
        android:text="@string/BUTTON_4"
        style="@style/Number"/>

    <!--5-->
    <Button
        android:id="@+id/button5"
        android:layout_toRightOf="@id/button4"
        android:layout_alignBaseline="@id/button4"
        android:text="@string/BUTTON_5"
        style="@style/Number"/>

    <!--6-->
    <Button
        android:id="@+id/button6"
        android:layout_toRightOf="@id/button5"
        android:layout_alignBaseline="@id/button5"
        android:text="@string/BUTTON_6"
        style="@style/Number"/>

    <!--7-->
    <Button
        android:id="@+id/button7"
        android:layout_below="@id/button4"
        android:text="@string/BUTTON_7"
        style="@style/Number"/>

    <!--8-->
    <Button
        android:id="@+id/button8"
        android:layout_toRightOf="@id/button7"
        android:layout_alignBaseline="@id/button7"
        android:text="@string/BUTTON_8"
        style="@style/Number"/>

    <!--9-->
    <Button
        android:id="@+id/button9"
        android:layout_toRightOf="@id/button8"
        android:layout_alignBaseline="@id/button8"
        android:text="@string/BUTTON_9"
        style="@style/Number"/>

    <!--0-->
    <Button
        android:id="@+id/button0"
        android:layout_below="@id/button7"
        android:text="@string/BUTTON_0"
        style="@style/Zero"/>

    <!--Plus-->
    <Button
        android:id="@+id/button_plus"
        android:layout_toRightOf="@id/button3"
        android:text="@string/BUTTON_PLUS"
        style="@style/Operator"/>

    <!--Minus-->
    <Button
        android:id="@+id/button_minus"
        android:layout_toRightOf="@id/button6"
        android:layout_alignBaseline="@id/button6"
        android:text="@string/BUTTON_MINUS"
        style="@style/Operator"/>

    <!--Multiply-->
    <Button
        android:id="@+id/button_multiply"
        android:layout_toRightOf="@id/button9"
        android:layout_alignBaseline="@id/button9"
        android:text="@string/BUTTON_MULTIPLY"
        style="@style/Operator"/>

    <!--Modulus-->
    <Button
        android:id="@+id/button_modulus"
        android:layout_toRightOf="@id/button0"
        android:layout_alignBaseline="@id/button0"
        android:text="@string/BUTTON_MODULUS"
        style="@style/Operator"/>

    <!--Divide-->
    <Button
        android:id="@+id/button_divide"
        android:layout_toRightOf="@id/button_modulus"
        android:layout_alignBaseline="@id/button_modulus"
        android:text="@string/BUTTON_DIVIDE"
        style="@style/Operator"/>

    <!--Clear-->
    <Button
        android:id="@+id/button_clear"
        android:layout_below="@id/button0"
        android:text="@string/BUTTON_CLEAR"
        style="@style/Clear"/>

    <!--Equals-->
    <Button
        android:id="@+id/button_equals"
        android:layout_toRightOf="@id/button_clear"
        android:layout_alignBaseline="@id/button_clear"
        android:text="@string/BUTTON_EQUALS"
        style="@style/Equals"/>
</RelativeLayout>

dimens.xml

<resources>
    <dimen name="button_margin">5dp</dimen>
    <dimen name="button_dimensions">60dp</dimen>
    <dimen name="button_text_size">30sp</dimen>
    <dimen name="zero_button_width">130dp</dimen>
    <dimen name="equals_button_width">200dp</dimen>
    <dimen name="display_padding">20dp</dimen>
</resources>

styles.xml

<resources>
    <style name="BaseButton">
        <item name="android:layout_width">@dimen/button_dimensions</item>
        <item name="android:layout_height">@dimen/button_dimensions</item>
        <item name="android:layout_margin">@dimen/button_margin</item>
        <item name="android:textSize">@dimen/button_text_size</item>
        <item name="android:textColor">@android:color/white</item>
    </style>

    <!--Number-->
    <style name="Number" parent="BaseButton">
        <item name="android:background">@color/brown</item>
    </style>

    <!--Zero-->
    <style name="Zero" parent="Number">
        <item name="android:layout_width">@dimen/zero_button_width</item>
    </style>

    <!--Operator-->
    <style name="Operator" parent="BaseButton">
        <item name="android:background">@color/green</item>
    </style>

    <!--Clear-->
    <style name="Clear" parent="BaseButton">
        <item name="android:background">@color/red</item>
    </style>

    <!--Equals-->
    <style name="Equals" parent="BaseButton">
        <item name="android:layout_width">@dimen/equals_button_width</item>
        <item name="android:background">@color/blue</item>
    </style>
</resources>