how many four-digit, positive integers are there where each digit is a prime number?
Well, I can only think of 4 prime numbers: 2, 3, 5, 7
2357 =4! =24
{2, 3, 5, 7} | {2, 3, 7, 5} | {2, 5, 3, 7} | {2, 5, 7, 3} | {2, 7, 3, 5} | {2, 7, 5, 3} | {3, 2, 5, 7} | {3, 2, 7, 5} | {3, 5, 2, 7} | {3, 5, 7, 2} | {3, 7, 2, 5} | {3, 7, 5, 2} | {5, 2, 3, 7} | {5, 2, 7, 3} | {5, 3, 2, 7} | {5, 3, 7, 2} | {5, 7, 2, 3} | {5, 7, 3, 2} | {7, 2, 3, 5} | {7, 2, 5, 3} | {7, 3, 2, 5} | {7, 3, 5, 2} | {7, 5, 2, 3} | {7, 5, 3, 2} (total: 24)
Well, I can only think of 4 prime numbers: 2, 3, 5, 7
2357 =4! =24
{2, 3, 5, 7} | {2, 3, 7, 5} | {2, 5, 3, 7} | {2, 5, 7, 3} | {2, 7, 3, 5} | {2, 7, 5, 3} | {3, 2, 5, 7} | {3, 2, 7, 5} | {3, 5, 2, 7} | {3, 5, 7, 2} | {3, 7, 2, 5} | {3, 7, 5, 2} | {5, 2, 3, 7} | {5, 2, 7, 3} | {5, 3, 2, 7} | {5, 3, 7, 2} | {5, 7, 2, 3} | {5, 7, 3, 2} | {7, 2, 3, 5} | {7, 2, 5, 3} | {7, 3, 2, 5} | {7, 3, 5, 2} | {7, 5, 2, 3} | {7, 5, 3, 2} (total: 24)