Question

Find the total number of three-digit prime numbers that can be formed using the digits 1, 7 and 9. You may repeat digits.

Answer 1

There are only 3^3 = 27 numbers to choose from:

111, 117, 119, ... 991, 997, 999
So check each for primality.
since 111 = 3*37, none of the numbers 111,333,999 can be prime
Maybe you can think of some other shortcuts, such as casting out 3's or 9's.