#### Answer

(a) $f = 1.5\times 10^{9}~Hz$
(b) $\lambda = 1.0\times 10^{-6}~m$

#### Work Step by Step

(a) We can find the frequency of an electromagnetic wave with a wavelength of 20 cm.
$f = \frac{v}{\lambda}$
$f = \frac{3.0\times 10^8~m/s}{0.20~m}$
$f = 1.5\times 10^{9}~Hz$
(b) We can find the wavelength of a sound wave in water which has this frequency.
$\lambda = \frac{v}{f}$
$\lambda = \frac{1500~m/s}{1.5\times 10^{9}~Hz}$
$\lambda = 1.0\times 10^{-6}~m$