(c) Let $n$ be the frequency of fork $C$ then
$n_{A} = n + \frac{3n}{100} = \frac{103n}{100}$ and $n_{B} = n - \frac{2n}{100} = \frac{98n}{100}$
But $n_{A} - n_{B} = 5 \Rightarrow \frac{5n}{100} = 5 \Rightarrow n = 100\ Hz$
$\therefore n_{A} = \frac{(103)(100)}{100} = 103\ Hz$