The marks of 40 students are given as a 'less than' table: less than 10: 5, less than 20: 14, less than 30: 26, less than 40: 36, less than 50: 40. Find the median mark.

Convert to class frequencies by differencing: 5, 9, 12, 10, 4. With n = 40, n/2 = 20 and the cumulative counts 5, 14, 26 show the median class is 20–30, where l = 20, cf = 14, f = 12, h = 10. Median = 20 + ((20 − 14)/12) × 10 = 20 + 5 = 25.