If the frequency of a wave is increased by 25%, then the change in its wavelength will be: (medium not changed)

Since, the medium has not changed, speed of wave remains same.

$\Rightarrow \mathrm{v} = \mathrm{f\lambda } = \mathrm{constant}$

${\mathrm{f}}_1{\mathrm{\lambda }}_1 = {\mathrm{f}}_2{\mathrm{\lambda }}_2$

$\Rightarrow {\mathrm{f}}_1{\mathrm{\lambda }}_1 = (1.25{\mathrm{f}}_1){\mathrm{\lambda }}_2 (\because \mathrm{frequency} \mathrm{increased} \mathrm{by} 25\%)$

${\mathrm{\lambda }}_2 = \frac{{\mathrm{\lambda }}_1}{1.25} \Rightarrow {\mathrm{\lambda }}_2 \mathrm{decreases}.$

% change in wavelength = $\frac{{\mathrm{\lambda }}_1-{\mathrm{\lambda }}_2}{{\mathrm{\lambda }}_1}\times 100$

$\frac{{\mathrm{\lambda }}_1-{\displaystyle \frac{{\mathrm{\lambda }}_1}{1.25}}}{{\mathrm{\lambda }}_1}\times 100 = \frac{0.25}{1.25}\times 100 = \frac{100}{5} = 20\%$