Three sections wrote the same test. Section A: students 20, mean mark 72. Section B: students 30, mean mark 65. Section C: students 50, mean mark 58. What is the mean mark of all 100 students?

Total marks = 20(72) + 30(65) + 50(58) = 1440 + 1950 + 2900 = 6290, shared over 100 students, so the mean is 62.9. Averaging the three section means as (72 + 65 + 58)/3 = 65 ignores that the sections have very different sizes.