Ans is (48)
Let the lengths of tracks A and B be $x,y$ respectively.
Speed of John and Mary in m/s : $\frac{5}{3},\frac{25}{12}\;\left(\text{by multiplying speed in km/h by}\; \frac{5}{18} \right)$
Given $x+y=325$ and $\frac{9x}{\frac{5}{3}}=\frac{5y}{\frac{25}{12}}$
Solving these two equations , we get $x=100\;\text{mtr}$
$\therefore$ Time taken by Mary to complete track A : $\frac{100\;\text{mtr}}{\frac{25}{12}\text{m/s}}=48$ seconds.