Two taps X and Y can fill a tank in 10 hrs. and 15 hrs.respectively.  If the both taps are opened together, the tank will be full in_________?

A. 3 hrs.
B. 4 hrs,
C. 5 hrs.
D. 6 hrs.

X’s 1 hour work = 1/10
Y’s 1 hour work = 1/15
(x+y)’s 1 hour work = 1/10+1/15=5/30=1/6
Both the taps can fill the tank in 6 hrs.