The correct answers are:
7, 8 and 9.
Step-by-step explanation:
We can check divisibility using the rules of divisibility. We will start with the largest single-digit number, 9. The rule for divisibility by 9 is that the sum of the digits is divisible by 9. The digits of 504 add to 5+0+4=9; since 9 is divisible by itself, 504 is divisible by 9.
504/9 = 56; this means we need two single digit numbers that are factors of 56. 56 = 7*8; this means 7 and 8 are our other two numbers.