An escalator moves towards the top level at the rate of 7 ft.sec and its length is 180 feet. If a person walks on the moving escalator at the rate of 2 feet per second towards the top level, how much time does he take to cover the entire length.  Choose the correct choice.
A) 5 sec  B) 25 sec  C) 20 sec  D) 15 sec  E) 10 sec

Rationale: "explanation : time taken to cover the entire length = tot . dist / resultant speed = 180 / ( 7 + 2 ) = 20 sec answer : c"
Answer: C