How much time would it take him to walk up the moving escalator?

Question

A boy walks up a stalled escalator in 90 seconds. When the same escalator moves, he is carried up in 60 seconds. How much time would it take him to walk up the moving escalator?               

         (a) 48 seconds                (b) 36 seconds

         (c) 30 seconds                (d) 24 seconds