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math
Question by worker5537 (37)
For example: If you wanted to know 20 x 21, you'd quickly multiply 20 x 20 in your head and then add one more 20. What is that called?
Answer by gigo (1706)
In germany we call this head backing (Kopfstütze) like the thing that saves your head if you have a car crash.
Answer by esteban (1334)
When you use numbers that are close to the given numbers to estimate an answer, you are using "friendly numbers" they are sometimes called "amicable numbers. "
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