Math CentralQuandaries & Queries


Question from Larry, a parent:


I am trying to help my daughter to solve this math problem. But, I couldn't. Please help. The math problem is.

Lee counted by 7's beginning with one of the whole numbers from 1 through 7, until Lee passed 1000. If Lee counted
three of the following numbers, which number did Lee NOT count?

a. 107
b. 184
c. 534
d. 641

Thank you,

We have two responses for you

Hi Larry,

Let's try an experiment. Suppose Lee had started with 5 then he would count

5, 12, 19, 26, 33, ...

Divide each of the numbers in his sequence by 7 and look at the remainders. Do you see why this happens?



Model Lee's counting by doing the same thing yourself (you may stop earlier!) Write down the numbers you obtain. Find the differences of a few pairs. Notice anything?

So which three of the numbers above could come from such a sequence?

Good Hunting!

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