JAMB 1994 · UME · Q47

Estimate the median of the frequency distribution below.

Class interval	1–5	6–10	11–15	16–20	21–25
Frequency	6	15	20	7	2

Worked solution (try it first)

The cumulative frequencies are 6, 21, 41, 48, 50, so $N = 50$ and the median is the 25th value.
The 25th value lies in the class 11–15 (running total 21 to 41).
Its lower boundary is 10.5, its frequency is 20 and its width is 5.
Median $= 10.5 + \frac{25 - 21}{20} \times 5$ , which is $10.5 + 1 = 11\frac12$ , option B.