# Example: Mentalist Trick (2)

(related to Part: Tricks for Mental Maths)

You can use your mentalist power to impress people by guessing the number they have thought:

• Ask somebody to think a number between $$1$$ and $$100$$. (e.g. $$41$$)
• Ask your partner to double the number. (e.g. $$41\cdot 2=82$$)
• Next, she/he should add $$5$$ to the result. (e.g. $$82 + 5=87$$)
• Next, she/he should multiply the result by $$10$$ and tell you the result. (e.g. $$87\cdot 10=870$$)

### Finding out the number thought by your partner

Now, pretend to use your "mentalist powers" and without explaining, what you are doing, perform the following calculations:

• Calculate the half the result mentally. (e.g. $$870 / 2=435$$)
• Cancel the last digit (e.g. $$43\not 5=43$$)
• Subtract $$2$$ and tell the result (e.g. $$43 - 2=41$$)

Think about, why this trick always works. If you have a solution, edit this article correspondingly, or provide a comment.

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