Pipe A and pipe B fill water into a tank of capacity 1000 litres, at a rate of 200 l/min and 50 l/min. Pipe C drains at a rate of 25 l/min. Pipe A is open for 1 min and closed, then pipe B is open for 2 min and closed. Further the pipe c is opened and drained for another 2 min. This process is repeated until the tank is filled. How long will it take to fill the tank?	Pick one.
A) 15
B) 17
C) 18
D) 56
E) 20

Rationale: "tank capacity : 1000 l , 1 st - 200 l / min for 1 min , volume filled : 200 l 2 nd - 100 l / min for 2 min , volume filled : 100 l 3 rd ( water draining ) : 25 l / min * 2 : 50 l total : ( 200 + 100 ) - 50 = 250 l filled for 1 cycle number of 250 in 1000 l tank : 1000 / 250 = 4 time taken to fill : 4 * total time = 4 * 5 = 20 ( option e )"
Answer: E