Find the gcd and lcm of 20 and 56

By gcd, I think you mean gcf (Greatest Common Factor).

To find the gcf find all the factors of each number:

Factors of 20: 1, 2, 4,5,10, 20

Factors of 56: 1, 2, 4, 7, 8, 14, 28, 56

The largest common factor is 4.

LCM = Least Common Multiple

This is the smallest number that both numbers divide into evenly

Find the prime factors of each number:

Prime factors of 20: 2 * 2 * 5

Prime factors of 56: 2 * 2 * 2 * 7

To find the LCM, multiply all prime factors the most number of times they occur.

In the prime factor of 56, 2 appears 3 times and 7 appears once.

In the prime factor of 20 5 appears once.

LCM = 2 * 2 * 2 * 7 * 5 = 280