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Simple number trick (Posted on 2016-06-29) Difficulty: 1 of 5
A. Have someone write down a 3 digit number.

B. You write down the same number.

C. Have them write down another 3 digit number. (Don't let them write 999.)

D. You write down the number whose digits are 9-complements to theirs. i.e. If their first digit is 7 your first digit is 2. Try not to let them notice this.

Astound them by quickly computing (A*B)+(C*D).

What's your secret?

See The Solution Submitted by Jer    
Rating: 2.5000 (2 votes)

Comments: ( Back to comment list | You must be logged in to post comments.)
Solution Likely answer | Comment 3 of 5 |
In that case, I assume that the author meant (A*C)+(B*D) = A*(C+D) = A*999 = A*1000 - A, which I can do in my head!
  Posted by Steve Herman on 2016-06-30 08:22:49
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