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Algebra Difficulty 1.8 Junior Find the answer

How many integers between 100 and 300 are multiples of both 5 and 7, but are not multiples of 10?

A number or a short expression. Spacing and $ signs are ignored.

Solution

The integers that are multiples of both 5 and 7 are the integers that are multiples of 35. The smallest multiple of 35 greater than 100 is 3imes35=1053 imes 35=105. Starting at 105 and counting by 35s, we obtain 105, 140, 175, 210, 245, 280, 315. The integers in this list that are between 100 and 300 and are not multiples of 10 (that is, whose units digit is not 0) are 105,175,245105, 175, 245, of which there are 3.

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.