android:layout_width="100%"

<android.support.percent.PercentRelativeLayout
  xmlns:android="http://schemas.android.com/apk/res/android"
  xmlns:app="http://schemas.android.com/apk/res-auto"
  android:layout_width="match_parent"
  android:layout_height="match_parent">

  <TextView
    android:layout_height="wrap_content"
    app:layout_widthPercent="35%"
    app:layout_heightPercent="10%"
    app:layout_marginLeftPercent="10%"
    app:layout_marginStartPercent="10%"
    android:text="Some text" />

  <TextView
    android:layout_height="wrap_content"
    android:layout_alignParentEnd="true"
    android:layout_alignParentRight="true"
    android:padding="8dp"
    android:text="Another some text"
    app:layout_marginEndPercent="10%"
    app:layout_marginRightPercent="10%"
    app:layout_widthPercent="35%" />
 </android.support.percent.PercentRelativeLayout>

<android.support.percent.PercentFrameLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <Button
        android:background="#f0f0f0"
        app:layout_widthPercent="50%"
        app:layout_heightPercent="50%"
        app:layout_marginTopPercent="25%"
        app:layout_marginLeftPercent="25%" />
</android.support.percent.PercentFrameLayout>

<LinearLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:orientation="horizontal" >

<!-- Эта кнопка занимает 30% от всей ширины -->
<Button
    android:layout_width="0dp"
    android:layout_height="wrap_content"
    android:layout_weight="0.3" />

<!-- Эта - 50% -->
<Button
    android:layout_width="0dp"
    android:layout_height="wrap_content"
    android:layout_weight="0.5" />

<!-- А эта - 20% -->
<Button
    android:layout_width="0dp"
    android:layout_height="wrap_content"
    android:layout_weight="0.2" />

</LinearLayout>