One tap can fill a cistern in 2 hours and another tap can empty the cistern in 3 hours. How long will they take to fill the cistern if both the taps are opened ?

A. 5 hrs   
B. 7 hrs
C. 6 hrs   
D. 8 hrs
Answer:  C

Explanation:     

Net part filled in 1 hour = (1/2) - (1/3) = 1/6
Cistern will be full in 6 hours.