Tamim has a certain average for 9 innings. In the tenth innings, he scores 100 runs thereby increasing his average by 8 runs. His new average is:

Question: Tamim has a certain average for 9 innings. In the tenth innings, he scores 100 runs thereby increasing his average by 8 runs. His new average is:

Solution:
Let Tamim's average be x for 9 innings.
So, Tamim scored 9x runs in 9 innings.

In the 10th inning, he scored 100 runs then the average became (x + 8).
And he scored (x + 8) × 10 runs in 10 innings.

ATQ,
9x + 100 = 10 × (x + 8)
or, 9x + 100 = 10x + 80
or, x = 100 - 80
or, x = 20

New average = (x + 8) = 28 runs.