Write the factor which is common to the numbers 20, 10, 7, and 5.
Factors of 20 = 2 × 2 × 5 × 1
Factors of 10 = 2 × 5 × 1
Factors of 7 = 7 × 1
Factors of 5 = 5 × 1
So the only common factor is 1.