In a series of tests, A appeared for 8 tests. Each test was marked out of 30 and averages 25. However, while checking his files, A could only find 7 of the 8 tests. For these he scored 29, 26, 18, 20, 27, 24 and 29. Determine how many marks he scored for the eighth test.

Total number of tests = 8

The average score of A = 25

Let the score of the 8^th test be a.

Mean = $\dfrac{\text{Sum of all observations }}{\text{Number of all observations }}$

⇒ 25 = $\dfrac{29 + 26 + 18 + 20 + 27 + 24 + 29 + a}{8}$

⇒ 25 = $\dfrac{173 + a}{8}$

⇒ 173 + a = 25 x 8

⇒ 173 + a = 200

⇒ a = 200 - 173

⇒ a = 27

Hence, A scored 27 marks in the eighth test.