Question

Q. When your doing a multiplication problem, for example 84 times 37 = 3,108, how do you check the answer without a calculator?
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A. I'm kinda confused on how to check the answer, do i do 84 times 400??

Answer 1

here is a very old trick:
add up the digits of each factor
87---> 8+4 = 12 , do it again --- 1+2 = 3
37 ----> 3+7 = 10 ---> 1+0=1
multiply them : 3*1 = 3

take the answer and add up the digits
3108 ---> 3+1+0+8 = 12 -- 1+2 = 3

same result!, extremely high probability that our answer is correct

This is called "casting out nines"