Multiple choice

Sam has a dance class every third day and a swimming class every fifth day. If he attended both classes today, after how many days will he have both classes together?

  1. 15

  2. 20

  3. 30

  4. None
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

This is an LCM problem. Both classes happen together at times that are common multiples of 3 and 5. Since 3 and 5 are coprime, LCM(3, 5) = 3 × 5 = 15. So both classes will occur together again after 15 days (and then every 15 days). Option A is correct.

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