Method 1:

List the factors of each number.

Factors of 20: 2, 4, 5, 10, 20. \ \

Factors of 36: 2, 3, 4, 6, 9, 12, 18, 36.

Factors of 48: 2, 3, 4, 6, 8, 12,16, 24, 48.

Identify the greatest common factor.

The greatest common factor of 20, 28 and 36 is 4.

Method 2:

Write the prime factorization of each  number.

20 =  2 × 2 × 5.

28 =  2 × 2 × 7.

36=  2 ×  2 × 3 × 3.

48 =  2 × 2 × 2 × 2 × 3.

The GCF is the product of the common prime factors in each number.

GCF = 2 × 2  = 4.

The greatest common factor of 20, 28 and 36 is 4. \ \