Question

Ann and Makiko like to swim laps at the community center pool.

They are swimming together today (day 1), but they are on different swim schedules. Ann swims every three days and Makiko swims every five days.

Question: How many times will they both swim at the pool on the same day
during the next ten weeks (including the first day)?

please explain process and show thinking in steps.

Answer 1

Answer:   5

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Explanation:

The LCM of 3 and 5 is 3*5 = 15.

Every 15 days, the two people will swim together on the same day.

• Day 1 = first day swimming together
• Day 1+15 = Day 16 = second day swimming together
• Day 16+15 = Day 31 = third day swimming together

And so on.

Keep this process going of adding on 15 to find this sequence

1, 16, 31, 46, 61, 76, ...

We stop the sequence once we either reach 70 or go over it because there are 7*10 = 70 days in ten weeks. This assumes we use a 7 day week rather than a 5 day week.

As shown in bold, there are 5 items smaller than 70. So the two people swam together a total of 5 times

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Alternatively, you can solve 15n+1 = 70 for n to get n = 4.6 which rounds down to n = 4 to represent the set {0,1,2,3,4} which has 5 items in it to represent the five times total they swim together.

• n = 0 corresponds to day 15n+1 = 15(0)+1 = 1
• n = 1 corresponds to day 15n+1 = 15(1)+1 = 16

and so on until reaching n = 4 corresponding to day 15(4)+1 = 61.