Given

FM broadcast

Modulating frequency $=20 \mathrm{kHz}=f$

Deviation ratio $=\frac{ \mathrm{frequency} \mathrm{deviation} }{ \mathrm{modulating} \mathrm{frequency} }=\frac{∆f}{f}$

$\Rightarrow$ Frequency deviation, $\Delta f=f\times 10$

$=20 \mathrm{kHz}\times 10=200 \mathrm{kHz}$

$\Rightarrow$ Bandwidth $=2\left(f+\Delta f\right)$

$=2\left(20+200\right) \mathrm{kHz}$

$=440 \mathrm{kHz}$