A 'less than' table of scores reads — less than 20: 7, less than 40: 22, less than 60: k, less than 80: 55, less than 100: 60. Exactly 18 scores lie in the class 40–60. Find k.

A class frequency is the difference between the cumulative entries at its two ends, so k − 22 = 18 and k = 40. Subtracting the 18 from the entry above instead gives 55 − 18 = 37, and 55 − 22 = 33 counts the two classes 40–60 and 60–80 together. Checking with k = 40, the five class frequencies read 7, 15, 18, 15 and 5, which add to 60 — the total the table ends on.