Escalator problem

How to solve the following question A man can walk up a moving 'up' escalator in 30s. The same man can walk down this moving 'up' escalator in 90s. Assume that his walking speed is same upwards and downwards. How much time will he take to walk up the escalator, when its not moving? a. 30s b. 45s c. 60s d. 90s Thanks Praks