Preeti reached the metro station and found that the escalator was not working. She walked up the stationary escalator in time $t_1$. On other days, if she remains stationary on the moving escalator, then the escalator takes her up in time $t_2$. The time taken by her to walk up on the moving escalator will be

Question

$\begin{array}{1 1} (1) \frac{t_1+t_2}{2} \\ (2) \frac{t_1t_2}{t_2-t_1} \\ (3) \frac{t_1t_2}{t_1+t_2} \\ (4) t_1-t_2\end{array} $