Wheat yield (in quintals per hectare) on 84 farms:
10-15 -> 8, 15-20 -> 17, 20-25 -> 26, 25-30 -> 21, 30-35 -> 12.
Which number should be substituted for f (the frequency of the median class) in the median formula?

$n=84$ so $\frac{n}{2}=42$. Cumulative frequencies: 8, 25, 51, 72, 84; the first to reach 42 is 51, so the median class is 20-25 and f is its OWN frequency, 26. 17 is the frequency of the class before the median class — the previous row supplies cf, never f. 51 is the cumulative frequency of the median class, not its frequency. 42 is $\frac{n}{2}$ itself.