1. Take any two digit number.
2. Multiply by 15.
3. Multiply by 7.
4. Subtract 4 times your original number.

Why are you repeating yourself?

Start over
1. Take any two digit number.
2. Multiply by 13.
3. Multiply by 8.
4. Subtract 3 times your original number.


Why don't you use some algebra to prove it?
Then following the instructions above:
1. Let x = original number.
2. Multiply by 15:       15x
3. Multiply by 7:       105x
4. Subtract 4 times your original number.       105x - 4x = 101x
    (The 101 just moves the 2-digit number over 2 places and adds the number)

When you start over,
1. Let x = original number.
2. Multiply by 13:       13x
3. Multiply by 8:       104x
4. Subtract 3 times your original number.       104x - 3x = 101x
    (The 101 just moves the 2-digit number over 2 places and adds the number)