First factor the numerator and denominator.

12 =  2 · 2 · 3

40 =  2 · 2 · 2 · 5

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

GCF = 2 · 2 = 4 

Divide the numerator and the denominator by GCF.

                  (Simplify)

Simplifest form of the fraction is .