Number Trick
1. Think of a number.
2. Add 17.
3. Double.
4. Subtract 4.
5. Double.
6. Add 20.
7. Divide by 4.
8. Subtract 20.
9. What number do you have?
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You should have your original number back.
Why does this work?
Let's use algebra to explain it.
1. Think of a number.
Let x = the original number.
2. Add 17.
Then we get x + 17
3. Double.
We have 2 (x + 17) or 2x + 34
4. Subtract 4.
Now we get 2x + 30
5. Double.
Now we have 4x + 60
6. Add 20.
We obtain 4x + 80
7. Divide by 4.
We get x + 20
8. Subtract 20.
We get x, the original number