LESSON 2 Divisibility Rules

Is It Divisible by 2?

Shannon, Roberto, and Ming are talking about good books to read. Shannon shows Roberto and Ming the book she plans to read this weekend.

Shannon checked Ming's answer by multiplying 159 by 2 on the calculator. She got 318 as her answer: 159 2 = 318. So 2 is a factor of 318.

A number is divisible by 2 if dividing it by 2 gives a whole number. Since 318 ÷ 2 gives a whole number, 318 is divisible by 2. A number is always divisible by its factors.

Shannon plans to read 159 pages each day. What if the book Shannon plans to read had 319 pages?

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The “.5” in the calculator window tells us there is a remainder. If the book had 319 pages, Shannon could read 159 whole pages each day—Saturday and Sunday. But if she did that, she would only read 159 2, or 318, pages. There would be a remainder of one page. Shannon could divide the leftover page in half. (The decimal .5 is the same as .) She could read an extra page on both Saturday and Sunday.

Since 319 ÷ 2 does not give a whole number, 319 is not divisible by 2.