How many three digits numbers can be formed by using the digits ` 0,1,3,5,7` while each digit may be repeated any number of times?

Explanation

The hundred's place cannot contain 0. So, it can be filled by any of the digits 1,3,5,7.


So, there are 4 ways to fill it.


Now, each of the ten's and unit's digits can be filled by any of the given five digits.


So, each can be filled in 5 ways.


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