Monthly mobile data used (in GB) by 90 subscribers:
0-2 -> 11, 2-4 -> 19, 4-6 -> 24, 6-8 -> 21, 8-10 -> 15.
State the median class (you should not need the median formula for this).

Running totals: 11, 30, 54, 75, 90, so $n=90$ and $\frac{n}{2}=45$. The cumulative count first passes 45 in the third row, so the median class is 4-6. 2-4 is the last class whose cumulative total (30) is still below 45 — it supplies cf = 30, not the class. 6-8 is one row too far. 8-10 is the class in which the nth observation falls, not the $\left(\frac{n}{2}\right)$th.