How many four-digit, positive integers are there where each digit is a prime number?
I need to find four-digit numbers where each digit is a prime number. The prime numbers that are single digits are 2, 3, 5, and 7. So, for each place value, I have four choices of digits. Therefore, the total number of such numbers is 4^4, which is 256. Alternatively, I could have used the counting principle and multiplied the number of choices for each place value: 4 * 4 * 4 * 4 = 256. 