Answer:
64
Explanation:
There are 4 prime digits: 2, 3, 5, and 7.
If the number is one digit, there are 4 possible numbers.
If the number is two digits, there are 4(3)=12 possible numbers.
If the number is three digits, there are (4)(3)(2)=24 possible numbers.
If the number is four digits, there are (4)(3)(2)(1)=24 possible numbers.
Taking the sum, the answer is 64.