A batsman in his 18
th
innings makes a score of 80 and thereby increases his average by 4. What is his average after the 18
th
innings?

Let the batsman's average before the 18
th
innings (i.e., for 17 innings) be A.

Total runs scored in 17 innings =17A

In the 18
th
innings, he scores 80 runs. So, total runs after 18 innings:

Total runs=17A+80
His new average after 18 innings increases by 4, so it becomes A+4.
Therefore, total runs can also be written as:

Total runs=18(A+4)
Equating the two expressions for total runs:

17A+80=18(A+4)
17A+80=18A+72
80−72=18A−17A
A=8
The average before the 18
th
innings was 8.
The average after the 18
th
innings is:

New Average=A+4=8+4=12