RRB NTPC · Question 2
It takes 15 minutes to fill an oil tank. However the oil tank is being emptied through an exhaust pipe, which can empty it in 30 minutes. If this exhaust pipe remains open, how long will it take to fill this tank completely? A. 20 minutes B. 25 minutes C. 30 minutes D. 40 minutes
Show answer & explanation
Answer: (D) C
Explanation
Net filling rate = 1/15 − 1/30 = 1/30 of the tank per minute, so it takes 30 minutes to fill.