First factor the numerator and denominator.

40 = 2 · 2 · 2 · 5

100 = 2 · 2 · 5 · 5

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

GCF = 2 · 2 · 5 = 20 \ \

Divide the numerator and the denominator by GCF.

             (Divide)

Simpliest form of the fraction is .