Math CentralQuandaries & Queries


Subject: Mathematical array for 85
Name: Lou (for Matt)
Who are you: Parent

This Gr. 4 math problem is asking for an array as explained below. We can figure out an array for 60 but the problem asks that an array be created for any number between min. 60 to max. 85.

No one in the family can figure this one out... help/advice would be greatly appreciated

Gr.4 math question:
Create an array for a number (qty) of 60 to 85 marbles in rows and columns.
Each row has the same number marbles.
No row or column can exceed 9 (nine)

Thank you!

Hi Lou and Matt,

I think there is some confusion in what is being asked. You are to find some number between 60 and 85 for which you can create an array with at least 9 rows and 9 columns. You are not asked to do it for every number between 60 and 85 because for some numbers in that range you can't do it.

If you create an array then the number of rows times the number of columns gives you the number of marbles in the array. So, for example, if you create an array of 65 marbles it must have

  • 1 row and 65 columns, or
  • 65 rows and 1 column, or
  • 5 rows and 13 columns, or
  • 13 rows and 5 columns.

I know this because the only way you can write 65 as the product of two numbers is 1 x 65, 65 x 1, 5 x 13 or 13 x 5. You can find some other examples in my answer to an earlier question.

It would be fun to actually do this, but I wouldn't use marbles since it is too easy for them to roll away. You could use pennies or m & m candies (or smarties if you are in Canada) or something where you have 85 identical things. Now form an array with some of them, not necessarily all of them but at least 60. The array can't have more than 9 rows and it can't have more than 9 columns.

If you do this then let me know how many you used and how many rows and columns.

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