Table: Reviewees who passed a mock exam — Group A 36 of 45, Group B 42 of 60, Group C 27 of 30. What was the overall passing rate of all three groups combined?

The correct answer is B. about 78%

Combined passers = 36 + 42 + 27 = 105, out of 45 + 60 + 30 = 135 reviewees. Rate = 105 ÷ 135 × 100 = 77.78%, about 78%. The simple average of 80, 70 and 90 (80%) is wrong because the groups differ in size.