The typing speed (words per minute) of 60 trainees at a Mysuru institute:
20-30 -> 7, 30-40 -> 11, 40-50 -> 12, 50-60 -> 16, 60-70 -> 14.
Find the median typing speed.

Cumulative frequencies: 7, 18, 30, 46, 60, so $n=60$ and $\frac{n}{2}=30$. Here $\frac{n}{2}$ lands exactly on a cumulative total, so the median sits on the shared boundary 50 and either reading of the median class gives the same value. Taking 50-60 (first cf strictly greater than 30): $50+\left(\frac{30-30}{16}\right)\times 10=50$. Taking 40-50 (first cf reaching 30): $40+\left(\frac{30-18}{12}\right)\times 10=40+10=50$. The distractor 40 comes from pairing $l=40$ with cf = 30, the cumulative frequency of that class ITSELF rather than of the class before it. 47.5 mixes rows the other way, keeping $l=40$ and cf = 18 but taking $f=16$ from the next class. 55 is the class mark of 50-60, and a class mark is not a median.