Answer
(a) $f' = 1790~Hz$
(b) $f' = 1510~Hz$
Work Step by Step
(a) We can find the frequency we detect as:
$f' = (1+\frac{v_{obs}}{v_{snd}})~f$
$f' = (1+\frac{30~m/s}{343~m/s})~(1650~Hz)$
$f' = 1790~Hz$
(b) We can find the frequency we detect as:
$f' = (1-\frac{v_{obs}}{v_{snd}})~f$
$f' = (1-\frac{30~m/s}{343~m/s})~(1650~Hz)$
$f' = 1510~Hz$
