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SOURCE: http://www.wisebread.com/11-useful-genius-math-tricks-that-are-actually-easy

If you need to be able to decide quickly if 408 slices of pie can be evenly split by 12 people, here are some useful shortcuts. These rules works for all numbers without fractions and decimals.

- Divisible by 2 if the number’s last digit is divisible by 2 (e.g. 298).
- Divisible by 3 if the sum of the digits of the number are divisible by 3 (501 is because 5 + 0 + 1 equals 6, which is divisible by 3).
- Divisible by 4 if the last two digits of the number are divisible by 4 (2,340 because 40 is a multiple of 4).
- Divisible by 5 if the last digit is 0 or 5 (1,505).
- Divisible by 6 if the rules of divisibility for 2 and 3 work for that number (408).
- Divisible by 9 if the sum of digits of the number are divisible by 9 (6,390 because 6 + 3 + 9 + 0 equals 18, which is divisible by 9).
- Divisible by 12 if the rules of divisibility for 3 and 4 work for that number (e.g. 408).

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