For the grouped data 0–10: 8, 10–20: 15, 20–30: 25, 30–40: 10, 40–50: 5 (class interval: frequency), find the mode using Mode = l + ((f1 − f0)/(2f1 − f0 − f2)) × h.

The modal class is 20–30 because its frequency 25 is the largest, so l = 20, f0 = 15, f1 = 25, f2 = 10 and h = 10. Mode = 20 + ((25 − 15)/(50 − 15 − 10)) × 10 = 20 + (10/25) × 10 = 24. The class mark 25 and the lower limit 20 are the two usual wrong picks.