Two taps A and B can fill a tank in 10 hours and 15 hours respectively. If both the taps are opened together, the tank will be full in:

A. 5 hrs   
B. 12 and 1/2 hrs
C. 6 hrs   
D. 7 and 1/2 hrs
Answer:     C

Explanation:     

A's hours work=1/10, B's 1 hours work = 1/15,
(A+B)s 1 hours work = (1/10) + (1/15) = 5/30 = 1/6
Both the taps can fill the tank in 6 hours.