Pipe A fills a tank of capacity 850 liters at the rate of 40 liters a minute. Another pipe B fills the same tank at the rate of 30 liters a minute. A pipe at the bottom of the tank drains the tank at the rate of 20 liters a minute. If pipe A is kept open for a minute and then closed and pipe B is open for a minute and then closed and then pipe C is open for a minute and then closed and the cycle is repeated, when will the tank be full? Choices.
A) 42 minutes	B) 51 minutes	C) 39 minutes	D) 40 minutes 20 seconds	E) none of these

Rationale: "in one cycle they fill 40 + 30 - 20 = 50 liters 850 = 50 * n = > n = 17 here n = number of cycles . total time = 17 * 3 = 51 as in one cycle there are 3 minutes . thus 51 minutes answer : b"
Answer: B