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4. Math > 37. Test for Divisibility

Test for Divisibility

It’s often useful to know whether one number is evenly divisible by another number. Here are some tricks that go beyond knowing whether a number is odd or even, or divisible by 10.

Before decimals such as 3.5 were invented, people had to use numbers with fractional parts instead, such as 31/2. In many division problems, they had to reduce fractions with large numbers—for example, 243 / 405—to their lowest terms—in this case, 3/5. Knowing rules to determine divisibility by the integers from 1 through 12, or from 1 through 15, was very useful in that precalculator time.1

If you want to strengthen your mental math powers, knowing the same rules can be useful to you today. In particular, these rules are helpful with math tricks that involve factoring numbers, such as simplified mental multiplication. Sometimes, knowing that a number is evenly divisible by another number goes at least halfway toward knowing what the answer is.


  

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