A batsman scored 51 runs in 13th innings such that his average decreased by 3

A batsman scored 51 runs in 13th innings such that his average decreased by 3. What was his average after 12 innings?

a. 90

b. 110

c. 80

d. 60

e. 130

Sol:

Let the average after 12th innings = n

Then the average after 13th innings = n – 3

12 * n + 51 = 13 * (n – 3)

51 + 39 = 13n – 12n

n = 90