Answer
$f' = 1126~Hz$
Work Step by Step
We can find the frequency produced by the whistle as:
$f = \frac{v_{snd}}{4L}$
$f = \frac{343~m/s}{(4)(0.0710~m)}$
$f = 1208~Hz$
We then find the frequency received by a stationary observer:
$f' = \frac{f}{1+\frac{v_{source}}{v_{snd}}}$
$f' = \frac{1208~Hz}{1+\frac{25~m/s}{343~m/s}}$
$f' = 1126~Hz$
