How can you show that the answer to 56x39 is exactly divisible by 21 without working it out?

To show that the product of 56 and 39 is exactly divisible by 21, you can check if both numbers are divisible by the prime factors of 21, which are 3 and 7. Since 56 is divisible by 7 (56 ÷ 7 = 8) and 39 is divisible by 3 (39 ÷ 3 = 13), their product will also be divisible by both 3 and 7. Therefore, 56 x 39 is divisible by 21.