The petrol (in litres) bought by each of 75 customers at a highway fuel pump one evening:
0-10 -> 6, 10-20 -> 13, 20-30 -> 21, 30-40 -> 20, 40-50 -> 15.
Find the median purchase (litres).

$n=75$, so $\frac{n}{2}=37.5$ — for grouped data we use $\frac{n}{2}$ even when $n$ is odd. Cumulative: 6, 19, 40, 60, 75. The first cf to reach 37.5 is 40, so the median class is 20-30 with $l=20$, cf = 19, $f=21,h=10$. Median = $20+\left(\frac{37.5-19}{21}\right)\times 10=20+8.81=28.81$ litres. 29.05 comes from using $\frac{n+1}{2}=38$, the ungrouped-data rule, which does not apply here. 18.81 uses cf = 40, the cumulative frequency of the median class itself, giving a negative correction. 20.88 forgets to multiply by $h=10$.