b
(b) Initially number of beats per second $= 5$
$ \therefore$ Frequency of pipe $= 200 \pm \,5 = 195 Hz$ or $205 Hz$ ...$(i)$
Frequency of second harmonics of the pipe $= 2n$
and number of beats in this case $= 10$
$ \therefore$ $2n = 420 \pm \, 10 $
==> $410 Hz$ or $430 Hz$
==> $n = 205 Hz$ or $215 Hz$ ... $(ii)$
From equation $(i)$ and $(ii)$ it is clear that $n = 205 Hz$