A) 16 min
B) 15 min
C) 20 min
D) 30 min
Correct Answer: B
$ \therefore $ Tank filled by 1st pipe in 1 min $ =\frac{1}{12} $ Now, 2nd pipe takes 20 min to empty the full tank.
$ \therefore $ Tank emptied by 2nd pipe in minute $ =\frac{1}{20} $ Now, time taken by both pipes to fill complete tank $ =\frac{1}{12}-\frac{1}{20}=\frac{5-3}{60}=\frac{2}{60}=\frac{1}{30} $ Since, total tank will be filled in 30 min. Hence, time taken to fill the half tank $ =\frac{30}{2}=15\min $