The fundamental frequency of a closed pipe is 220Hz. If $\frac{1}{4}$ of the pipe is filled with water, the frequency of the first overtone of the pipe now is

Fundamental frequency of closed pipe
$\mathrm{n} = \frac{\mathrm{v}}{4\mathrm{l}} = 220 \mathrm{Hz} \Rightarrow \mathrm{v} = 220 \times 4\mathrm{l}$
If $\frac{1}{4}$ of the pipe is filled with water then remaining length of air column is $\frac{3\mathrm{l}}{4}$
Now fundamental frequency = $\frac{\mathrm{v}}{4\left({\displaystyle \frac{3\mathrm{l}}{4}}\right)} =\frac{\mathrm{v}}{3\mathrm{l}} \mathrm{and}$
First overtone = 3 x fundamental frequency

                      = $\frac{3\mathrm{v}}{3\mathrm{l}} = \frac{\mathrm{v}}{\mathrm{l}} = \frac{220\times 4\mathrm{l}}{\mathrm{l}}= 880 \mathrm{Hz}$