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| Math Central | Quandaries & Queries |
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Question from Abbey, a student: Starting from the same spot, one boy completed a route in 9 minutes and another boy completed it in 15 minutes. When will both boys be at the same starting point at the same time again? |
Hi Abbey,
The first boy completes the route once in 9 minutes, twice in 18 minutes and so on. These numbers
9, 18, ...
are the multiples of 9.
The second boy completes the route once in 15 minutes, twice in 30 minutes and so on. These numbers
15, 30, ...
are the multiples of 15.
Extend the lists until you find the first number that is each list. That time will be the first time they are both back to the starting point. This number is called the least common multiple of 9 and 15.
Penny
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