1.  Three taps A, B and C can fill a tank in 12, 15 and 20 hours respectively. If A is open all the time and B and C are open for one hour each alternately, the tank will be full in:

A. 6 hours
B. 6(2/3) hours
C. 7 hours
D. 7(1/2) hours

Answer: Option C

Explanation:

(A + B)'s 1 hour's work = 1 + 1 = 9 = 3 .
12 15 60 20

(A + C)'s hour's work = 1 + 1 = 8 = 2 .
12 20 60 15

Part filled in 2 hrs = 3 + 2 = 17 .
20 15 60

Part filled in 6 hrs = 3 x 17 = 17 .
60 20

Remaining part = 1 - 17 = 3 .
20 20

Now, it is the turn of A and B 
and3part is filled by A and B in 1 hour.
20


Total time taken to fill the tank = (6 + 1) hrs = 7 hrs.