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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 over1. 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)