The average of a and b is (a + b)/2.

If we add 12a + 3b = 1 and 7b - 2a = 0 we get:

12a + 3b + 7b - 2a = 1 + 0

10a + 10b = 1

10(a + b) = 1 divide both sides by 10

a + b = 1/10 divide both sides by 2

(a + b)/2 = 1/(2*10)

(a + b)/2 = 1/20

The average of a and b is 1/20.