7. Preeti reached the metro station and found that the escalator was not working. She walked up the stationary escalator in time t1. On other days, if she remains stationary on the moving escalator. then the escalator takes her up in time t2. The time taken by her to walk up on the moving escalator will be:

7. Preeti reached the metro station and found that the escalator was not working. She walked up the stationary escalator in time t1. On other days, if she remains stationary on the moving escalator. then the escalator takes her up in time t2. The time taken by her to walk up on the moving escalator will be:

(a) \( \dfrac{t_1 t_2}{t_2 - t_1} \)

(b) \( \dfrac{t_1 t_2}{t_2 + t_1} \)

(c) \( t_1 - t_2 \)

(d) \( \dfrac{t_1 + t_2}{2} \)

Answer :  (b) \( \dfrac{t_1 t_2}{t_2 + t_1} \)

\( V_{\text{total}} = V_P + V_E \)

\( \dfrac{d}{t} = \dfrac{d}{t_1} + \dfrac{d}{t_2} \)

\( \dfrac{1}{t} = \dfrac{t_1 + t_2}{t_1 t_2} \)

\(t = \dfrac{t_1t_2}{t_1 + t_2} \)