Another trick: if the last 4 digits of the number are divisible by 16, the full number is divisible by 16. So far as I know, if the last X digits of a number are divisible by 2^X, the entire number is divisible by 2^X.
@James: nice trick! (and I guess the proof arises from 10^x being always exactly divisible by 2^x, so "iff" also applies?)
perhaps less interesting but I guess you could do the same with powers of 5, iff the last X digits of a number are divisible by 5^x, then the entire number is divisible by 5^x...
Add two numbers
given 3 sides, find area of this triangle
Magic is simple (for beginners)
Rule of mixtures (composites) - weighted bound
Rule of mixtures (composites) - lower bound
Inscribed Pentagon? 2
Divisible by 12
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