Math CentralQuandaries & Queries


Question from cindy, a parent:

Sandra is more than 20 years old and less than 60 years old. You can count by 7's to reach Sandra's age. Next year you will be able to count by 5's to reach Sandra's age. How old is Sandra?

Hi Cindy,

Sandra's age is more than 20 and it is a multiple of 7 so the youngest she could be is 21. If so then next year she will be 22 and 22 is not a multiple of 5. Thus Sandra's age is more than 21. What is the next multiple of 7? Is one more than that a multiple of 5? If not what is the next multiple of 7?


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