activity_main.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">


    <ImageButton
        android:id="@+id/quitButton"
        android:layout_width="48dp"
        android:layout_height="48dp"
        android:src="@drawable/quit_app_icon_bigger"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:background="@android:color/transparent"
        android:onClick="onQuitButtonClick"
        android:contentDescription="@+string/quit_button_description"/>


    <ImageButton
        android:id="@+id/settingsButton"
        android:layout_width="48dp"
        android:layout_height="48dp"
        android:src="@drawable/settings_gear_bigger"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        android:layout_marginEnd="8dp"
        android:layout_marginTop="8dp"
        android:background="@android:color/transparent"
        android:contentDescription="@+string/settings_button_description"/>


    <TextView
        android:id="@+id/currentmodetext"
        android:layout_width="300dp"
        android:layout_height="wrap_content"
        android:layout_margin="15dp"

        android:text="Formative Mode (40%)"
        android:textAlignment="center"
        android:textColor="@color/black"
        android:textSize="24dp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.555"

        app:layout_constraintStart_toStartOf="parent"
        android:layout_marginLeft="30dp"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@+id/inputext"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginBottom="30dp"

        android:text=""
        android:textAlignment="center"
        android:textColor="@color/black"
        android:textSize="115dp"
        app:layout_constraintBottom_toTopOf="@+id/calculatedgradetext"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.0"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/currentmodetext"
        app:layout_constraintVertical_bias="0.703" />

    <TextView
        android:id="@+id/calculatedgradetext"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_margin="18dp"

        android:layout_marginBottom="16dp"
        android:text="Grade: 0.0%"
        android:textAlignment="center"

        android:textColor="@color/black"
        android:textSize="31dp"
        app:layout_constraintBottom_toTopOf="@+id/historytext"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.482"
        app:layout_constraintStart_toStartOf="parent" />

    <TextView
        android:id="@+id/historytext"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="start"
        android:layout_margin="5dp"
        android:textSize="18sp"
        android:visibility="gone"
        android:text=""
        android:textColor="@color/historyColor"
        app:layout_constraintBottom_toTopOf="@id/line"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent" />


    <View
        android:id="@+id/line"
        android:layout_width="match_parent"
        android:layout_height="1dp"
        android:background="@color/linecolor"
        app:layout_constraintBottom_toTopOf="@id/linearLayout"
        android:layout_marginBottom="8dp"/>

    <LinearLayout
        android:id="@+id/linearLayout"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="vertical"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent">

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

            <com.google.android.material.button.MaterialButton
                style="@style/digitButton"
                android:id="@+id/btn_7"
                android:onClick="onDigitClick"
                android:text="7" />

            <com.google.android.material.button.MaterialButton
                style="@style/digitButton"
                android:id="@+id/btn_8"
                android:onClick="onDigitClick"
                android:text="8" />

            <com.google.android.material.button.MaterialButton
                style="@style/digitButton"
                android:id="@+id/btn_9"
                android:onClick="onDigitClick"
                android:text="9" />

            <com.google.android.material.button.MaterialButton
                style="@style/importantButtons"
                android:id="@+id/btn_delete"
                android:onClick="onbackClick"
                app:icon="@drawable/whitebackspace"
                app:iconTint="@color/white"

                android:text="4"/>

        </LinearLayout>

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

            <com.google.android.material.button.MaterialButton
                style="@style/digitButton"
                android:id="@+id/btn_4"
                android:onClick="onDigitClick"
                android:text="4" />

            <com.google.android.material.button.MaterialButton
                style="@style/digitButton"
                android:id="@+id/btn_5"
                android:onClick="onDigitClick"
                android:text="5" />

            <com.google.android.material.button.MaterialButton
                style="@style/digitButton"
                android:id="@+id/btn_6"
                android:onClick="onDigitClick"
                android:text="6" />

            <com.google.android.material.button.MaterialButton
                style="@style/importantButtons"
                android:id="@+id/btn_mode"
                android:onClick="onMClick"
                android:text="M" />

        </LinearLayout>

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

            <com.google.android.material.button.MaterialButton
                style="@style/digitButton"
                android:id="@+id/btn_1"
                android:onClick="onDigitClick"
                android:text="1" />

            <com.google.android.material.button.MaterialButton
                style="@style/digitButton"
                android:id="@+id/btn_2"
                android:onClick="onDigitClick"
                android:text="2" />

            <com.google.android.material.button.MaterialButton
                style="@style/digitButton"
                android:id="@+id/btn_3"
                android:onClick="onDigitClick"
                android:text="3" />

            <com.google.android.material.button.MaterialButton
                style="@style/PlusMinusButton"
                android:id="@+id/btn_plusMinus"
                android:onClick="onclearAllClick"
                android:text="AC" />

        </LinearLayout>

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

            <com.google.android.material.button.MaterialButton
                style="@style/digitButton"
                android:id="@+id/btn_0"
                android:onClick="onDigitClick"
                android:text="0" />

            <com.google.android.material.button.MaterialButton
                style="@style/digitButton"
                android:id="@+id/btn_period"
                android:onClick="onDigitClick"
                android:text="." />

            <com.google.android.material.button.MaterialButton
                style="@style/operatorButton"
                android:id="@+id/btn_equal"
                android:onClick="onequalClick"

                android:text="=" />

        </LinearLayout>

    </LinearLayout>
</androidx.constraintlayout.widget.ConstraintLayout>