An escalator moves towards the top level at the rate of 15 ft.sec and its length is 180 feet. If a person walks on the moving escalator at the rate of 3 feet per second towards the top level, how much time does he take to cover the entire length.	Choose the correct answer:  A) 12 sec  B) 20 sec  C) 15 sec  D) 18 sec  E) 10 sec

Rationale: "explanation : time taken to cover the entire length = tot . dist / resultant speed = 180 / ( 15 + 3 ) = 10 sec answer : e"
Answer: E