The marks of 150 candidates in a mock test are given as a 'less than' table:
less than 20 -> 12, less than 40 -> 38, less than 60 -> 82, less than 80 -> 125, less than 100 -> 150.
Find the median marks.

A 'less than' column is already cumulative, so the ordinary frequencies are the successive differences: 12, 26, 44, 43, 25 (total 150). With $\frac{n}{2}=75$, the first cumulative entry to reach 75 is 82, so the median class is 40-60 with $l=40$, cf = 38, $f=44,h=20$. Median = $40+\left(\frac{75-38}{44}\right)\times 20=40+16.82=56.82$. 71.44 is what you get by treating 12, 38, 82, 125, 150 as ordinary frequencies (total 407) — the single most common error with these tables. 36.82 uses cf = 82, the median class's own cumulative frequency, and 40.84 omits the factor $h=20$.