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

Let v1is the velocity of Preeti on stationary escalator and d is the distance travelled by her∴ v1=dt1 Again, let v2 is the velocity of escalator ∴ v2=dt2∴Net velocity of Preeti on moving escalator with respect to the groundv=v1+v2=dt1+dt2=dt1+t2t1t2The time taken by her to walk up on the moving escalator will bet=dv=ddt1+t2t1t2=t1t2t1+t2