i have 5 strings some are changing dynamically but some can't

actually i am new to programming, but i tried this for a simple math game. Here in this program i made s1,s2,s3,s4 & s5 to generate dynamically.. but only s1, s2 are changing, s3,s4,s5 are not changing if the method calls again... please help me.

public class MainActivity extends Activity implements OnClickListener {
Button b1, b2, b3;
ImageView i, i2, i3;
TextView t1, t2, t3;
String s1, s2, s3, s4, s5;

int m, r2, r3, r4, r5, r6, r7, r8, r9, r10, r11, r12, r13, n[];

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    b1 = (Button) findViewById(R.id.button1);
    b2 = (Button) findViewById(R.id.button2);
    b3 = (Button) findViewById(R.id.button3);

    i = (ImageView) findViewById(R.id.imageView1);
    t1 = (TextView) findViewById(R.id.textView1);
    t2 = (TextView) findViewById(R.id.textView2);
    t3 = (TextView) findViewById(R.id.textView3);
    b1.setOnClickListener(this);
    b2.setOnClickListener(this);
    b3.setOnClickListener(this);
    imp();

}

public void imp() {
    Random r1 = new Random();
    r2 = r1.nextInt(15) + 1;
    r3 = r1.nextInt(15) + 1;
    r4 = r2 + r3;
    r5 = r2 - r3;
    r6 = r2 * r3;

    n = new int[] { r4, r5, r6 };

    while (r9 == r11 || r9 == r13 || r11 == r13) {
        r8 = r1.nextInt(3) + 0;
        r9 = n[r8];
        r10 = r1.nextInt(3) + 0;
        r11 = n[r10];
        r12 = r1.nextInt(3) + 0;
        r13 = n[r12];
    }
    s1 = Integer.toString(r2);
    s2 = Integer.toString(r3);
    s3 = Integer.toString(r9);
    s4 = Integer.toString(r11);
    s5 = Integer.toString(r13);
    t1.setText(s1);
    t2.setText(s2);
    b1.setText(s3);
    b2.setText(s4);
    b3.setText(s5);

    int[] images = { R.drawable.plus, R.drawable.minus, R.drawable.multiply };

    Random random = new Random();
    m = images[random.nextInt(images.length)];
    i.setImageResource(m);
}

public void click(int k, Button b) {
    if (m == R.drawable.plus && k == r4 || m == R.drawable.minus && k == r5
            || m == R.drawable.multiply && k == r6) {

        imp();

    } else {
        t3.setText("FINISH");
        t3.setVisibility(View.VISIBLE);
        b.setPressed(true);
        b1.setClickable(false);
        b2.setClickable(false);
        b3.setClickable(false);

    }
}

@Override
public void onClick(View v) {
    switch (v.getId()) {
    case R.id.button1:
        click(r9, b1);

        break;
    case R.id.button2:
        click(r11, b2);
        break;
    case R.id.button3:
        click(r13, b3);
        break;
    }
  }
}

and the layout is

<LinearLayout xmlns:android="http://ift.tt/nIICcg"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:gravity="center_horizontal|center_vertical"
android:orientation="vertical" >

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

<TextView
    android:id="@+id/textView1"
    android:layout_width="wrap_content"
    android:layout_height="46dp"
    android:layout_weight="0.97"
    android:editable="false"
    android:text="Large Text"
    android:gravity="center"
    android:textAppearance="?android:attr/textAppearanceLarge" />

<ImageView
    android:layout_weight="1"
    android:id="@+id/imageView1"
    android:layout_width="67dp"
    android:layout_height="29dp"
    android:src="@drawable/plus" />

<TextView
    android:id="@+id/textView2"
    android:layout_width="wrap_content"
    android:layout_height="match_parent"
    android:layout_weight="1.00"
    android:text="Large Text"
    android:gravity="center"
    android:textAppearance="?android:attr/textAppearanceLarge" />

</LinearLayout>

<LinearLayout
    android:paddingTop="50dp"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:orientation="horizontal" >
 <Button
      android:layout_weight="1"
    android:id="@+id/button1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Button" />
<Button
     android:layout_weight="1"
    android:id="@+id/button2"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Button" />

<Button
     android:layout_weight="1"
    android:id="@+id/button3"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Button" />

</LinearLayout>

<TextView
android:id="@+id/textView3"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:paddingTop="50dp"
android:text="Large Text"
android:textAppearance="?android:attr/textAppearanceLarge"
android:textSize="50dp" 
android:visibility="invisible"/>

</LinearLayout>