In a grouped frequency distribution the class marks are 12, 20, 28, 36 and 44. Which class interval has 28 as its class mark?

Consecutive class marks differ by exactly the class size, so h = 20 − 12 = 8. A class mark is the midpoint of its class, so the class runs from 28 − 4 = 24 to 28 + 4 = 32. Writing 26–30 halves the class size a second time, 28–36 treats the class mark as the lower limit, and 20–36 stretches from one neighbouring class mark to the next, making the class twice as wide as it should be.