Answer : 3. "24min"
Explanation :

Answer: C) 24min Explanation: Given taps X and Y can fill the tank in 30 and 40 minutes respectively. Therefore,   part filled by tap X in 1 minute = 1/30   part filled by tap Y in 1 minute = 1/40   Tap Z can empty the tank in 60 minutes. Therefore,   part emptied by tap Z in 1 minute = 1/60   Net part filled by Pipes X,Y,Z together in 1 minute = [1/30 +1/40 - 1/60] = 5/120 = 1/24   i.e., the tank can be filled in 24 minutes.