need help with this question.

Walking down a certain escalator descending at a constant rate, if I take 30 seconds to reach the bottom, I have to walk down 13 consecutive steps. If i take 21 seconds to reach the bottom, I have to walk down 19 consecutive steps. The number of steps showing on this escalator at any one time is:

a) 32 b) 33 c) 34 d) 35 e) 36