activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="#7C616768"
    tools:context=".MainActivity">


    <TextView
        android:id="@+id/resultTextView"
        android:layout_width="100dp"
        android:layout_height="100dp"
        android:layout_marginStart="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginBottom="136dp"
        android:background="#4CAF50"
        android:clickable="false"
        android:gravity="center"
        android:text="C"
        android:textSize="50sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.498"
        app:layout_constraintStart_toStartOf="parent" />

    <TextView
        android:id="@+id/firstNumTextView"
        android:layout_width="75dp"
        android:layout_height="75dp"
        android:layout_marginStart="56dp"
        android:layout_marginTop="56dp"
        android:background="#2196F3"
        android:clickable="true"
        android:gravity="center"
        android:text="A"
        android:textSize="50sp"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@+id/operatorTextView"
        android:layout_width="75dp"
        android:layout_height="75dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="56dp"
        android:layout_marginEnd="8dp"
        android:background="#FF9800"
        android:clickable="true"
        android:gravity="center"
        android:text="+"
        android:textSize="50sp"
        app:layout_constraintEnd_toStartOf="@+id/secondNumTextView"
        app:layout_constraintStart_toEndOf="@+id/firstNumTextView"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:layout_width="75dp"
        android:layout_height="75dp"
        android:layout_marginStart="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginBottom="40dp"
        android:background="#AEB1B2"
        android:gravity="center"
        android:text="="
        android:textSize="75sp"
        app:layout_constraintBottom_toTopOf="@+id/resultTextView"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent" />

    <TextView
        android:id="@+id/secondNumTextView"
        android:layout_width="75dp"
        android:layout_height="75dp"
        android:layout_marginTop="56dp"
        android:layout_marginEnd="56dp"
        android:background="#2196F3"
        android:clickable="true"
        android:gravity="center"
        android:text="B"
        android:textSize="50sp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/goButton"
        android:layout_width="100dp"
        android:layout_height="75dp"
        android:layout_marginEnd="8dp"
        android:layout_marginBottom="8dp"
        android:text="GO!"
        android:textSize="30sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent" />
</android.support.constraint.ConstraintLayout>