Calculator XML

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.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"
    tools:context=".MainActivity">

    <EditText
        android:id="@+id/editTextNumber"
        android:layout_width="175dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="16dp"
        android:layout_marginTop="100dp"
        android:ems="10"
        android:inputType="number"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.0"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.007" />

    <EditText
        android:id="@+id/editTextNumber2"
        android:layout_width="175dp"
        android:layout_height="wrap_content"
        android:layout_marginTop="100dp"
        android:layout_marginEnd="16dp"
        android:ems="10"
        android:inputType="number"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.989"
        app:layout_constraintStart_toEndOf="@+id/editTextNumber"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.006" />

    <Button
        android:id="@+id/btnAdd"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="16dp"
        android:layout_marginTop="100dp"
        android:text="+"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/editTextNumber" />

    <Button
        android:id="@+id/btnSub"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="16dp"
        android:layout_marginTop="100dp"
        android:text="-"
        app:layout_constraintStart_toEndOf="@+id/btnAdd"
        app:layout_constraintTop_toBottomOf="@+id/editTextNumber" />

    <Button
        android:id="@+id/btnMul"
        android:layout_width="78dp"
        android:layout_height="52dp"
        android:layout_marginStart="16dp"
        android:layout_marginTop="100dp"
        android:text="*"
        app:layout_constraintStart_toEndOf="@+id/btnSub"
        app:layout_constraintTop_toBottomOf="@+id/editTextNumber2" />

    <Button
        android:id="@+id/btnDiv"
        android:layout_width="67dp"
        android:layout_height="50dp"
        android:layout_marginStart="16dp"
        android:layout_marginTop="100dp"
        android:layout_marginEnd="16dp"
        android:text="/"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toEndOf="@+id/btnMul"
        app:layout_constraintTop_toBottomOf="@+id/editTextNumber2" />

    <TextView
        android:id="@+id/txtAnswer"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="50dp"
        android:layout_marginTop="100dp"
        android:textSize="24sp"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/btnAdd" />
</androidx.constraintlayout.widget.ConstraintLayout>