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  3. A certain integer n is a multiple of both 5 and 9. Must n be a multiple of 15?

A certain integer n is a multiple of both 5 and 9. Must n be a multiple of 15?

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问题 A certain integer n is a multiple of both 5 and 9. Must n be a multiple of 15?
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答案 The first step in solving this problem is to find the Least Common Multiple (LCM) of 5 and 9. Because a multiple of 5 must have either a 5 or a 0 in the digits place, and 9×5 is the first multiple of 9 with a 5 in the digits place, 9×5=45 must be the LCM. Therefore, n must be a multiple of 15, since 45 (one possible value of n) is a multiple of 15.
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