The number of e-mails received per day by each of 70 employees of a firm, with the cumulative frequency column already filled in:
0-25 -> cf 9, 25-50 -> cf 23, 50-75 -> cf 41, 75-100 -> cf 58, 100-125 -> cf 70.
Which class is the median class?

The last cumulative frequency is $n$, so $n=70$ and $\frac{n}{2}=35$. Read down the cf column for the first entry that reaches 35: 9, then 23, then 41. So the median class is 50-75. Choosing 25-50 stops at cf 23, the row just BELOW $\frac{n}{2}$ — that row supplies the value of cf for the formula, it is not the median class itself. 75-100 counts one row too far, and 100-125 is where the cf reaches $n$, not $\frac{n}{2}$.