Math CentralQuandaries & Queries


Question from sherry, a parent:

I am a prime number. I can not be shared fairly except by one group. My array can only be one row.
array can only be one row. Can you please explain this out to me so I can help my child with it.

Hi Sherry,

Your child's teacher is using concepts that are familiar to her class to introduce the concept of a prime number. A prime number is an integer greater than 1 whose only positive factors are itself and one. For example 2, 3, 5, 7, 11 and 13 are prime numbers but 12 is not since 3 is a factor of 12.

Suppose your child has 7 gum drops and she wants to share them fairly with some friends who are coming over after school. If no one shows up but your daughter then she can eat all 7. If some friends do show up and there are 7 children in total then each can have one gum drop and they are shared fairly. For any other number of children the gum drops can no be shared fairly. (She could of course give one to her mother leaving 6 and then 6 could be shared fairly among 3 children, 2 each, or 2 children, 3 each.)

Similarly if you have 12 eggs you can put them in a 2 by 6 array. That's the way they come in the grocery store. You can also put them in a 3 by 4 array since 3 × 4 = 12. But if you have 13 eggs then the only possible arrays are either one row of 13 eggs or 13 rows of 1 egg each. If you are unfamiliar with the concept of arrays go to the Quandaries and Queries page of Math Central and use the Quick Search to search for the term array.

I hope this helps,

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