A large organ has a pipe 1.7 m long, closed at the top…

(a) $f_1 = \dfrac{340\ \text{m/s}}{4 \times 1.7\ \text{m}} = 50\ \text{Hz}$.

(b) The modes are $50\ \text{Hz}$, $150\ \text{Hz}$, $250\ \text{Hz}$ and so on, up to $950\ \text{Hz}$. That is $10$ frequencies.

(c) $f_1 = \dfrac{340\ \text{m/s}}{2 \times 1.7\ \text{m}} = 100\ \text{Hz}$, twice as high.