The distance (in km) travelled to school by 65 students of a Patna school:
0-2 -> 8, 2-4 -> 15, 4-6 -> 18, 6-8 -> 14, 8-10 -> 10.
Find the median distance (km).

Cumulative: 8, 23, 41, 55, 65, so $n=65$ and $\frac{n}{2}=32.5$. The first cf to reach 32.5 is 41, so the median class is 4-6 with $l=4$, cf = 23, $f=18$ and $h=2$. Median = $4+\left(\frac{32.5-23}{18}\right)\times 2=4+\frac{19}{18}=5.06$ km. 4.53 drops the factor $h=2$ — a very common slip when $h$ is small, because the answer still looks reasonable. 3.06 uses cf = 41, the median class's own cumulative frequency. 5.27 uses $f=15$, the frequency of the class BEFORE the median class.