The number of dropped calls per day logged at a mobile tower over 40 days:
0-5 -> 21, 5-10 -> 9, 10-15 -> 6, 15-20 -> 4.
Find the median number of dropped calls.

Cumulative: 21, 30, 36, 40, so $n=40$ and $\frac{n}{2}=20$. The very first cf, 21, already reaches 20, so the median class is the FIRST class 0-5. There is no class before it, so cf = 0 (not 21). With $l=0,f=21,h=5$: Median = $0+\left(\frac{20-0}{21}\right)\times 5=\frac{100}{21}=4.76$. Writing cf = 21, the median class's own cumulative frequency, would give a negative answer. 0.95 drops the factor $h=5$; 2.5 is the class mark of 0-5; 9.52 uses $h=10$ instead of the true class size 5.