Math CentralQuandaries & Queries


Question from sonia, a parent:

how many 4 digit odd numbers can be made using the digits 2, 5, 7, and 8

Hi Sonia,

You haven't said that the digits can't be repeated so I am going to assume they can be repeated. Thus. for example, 7275 is a number that meets your requirement.

Since you want an odd number the final digit must be 5 or 7. So use 2, 5, 7 and 8 to form a three digit number and then you can add either a 5 or 7 on the end to form a four digit number that meets your requirement. The answer then is twice the number of three digit numbers you can form from 2, 5, 7 and 8.


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