For the grouped data 15–20: 5, 20–25: 12, 25–30: 20, 30–35: 8, 35–40: 3 (class interval: frequency), find the mode.

The modal class is 25–30 with frequency 20, so l = 25, f0 = 12, f1 = 20, f2 = 8 and h = 5. Mode = 25 + ((20 − 12)/(40 − 12 − 8)) × 5 = 25 + (8/20) × 5 = 27. The lower limit 25 and the class mark 27.5 are the two common wrong picks.