A FM Broad cast transmitter, using modulating signal of frequency 20 kHz has a deviation ratio of 10. The Bandwidth required for transmission is : 

Given

FM broadcast

Modulating frequency = 20 k Hz = f

$$\begin{gathered} \mathrm{Deviation} \mathrm{ratio} = \frac{\mathrm{frequency} \mathrm{deviation}}{\mathrm{modulating} \mathrm{frequency}} = \frac{∆\mathrm{f}}{\mathrm{f}} \\ \Rightarrow \mathrm{frequency} \mathrm{deviation} =∆\mathrm{f} = \mathrm{f}\times 10 \\ = 20 \mathrm{kHz} \times 10 = 200 \mathrm{kHz} \\ \Rightarrow \mathrm{Bandwidth} = 2(\mathrm{f} + ∆\mathrm{f}) \\ = 2(20 + 200) \mathrm{kHz} \\ = 440 \mathrm{kHz} \end{gathered}$$