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 is? Choose the most appropriate option:
A) 6 hrs
B) 8 hrs
C) 7 hrs
D) 3 hrs
E) 12 hrs
(A + B)'s 1 hour work = (1/12 + 1/15) = 3/20   (A + C)'s 1 hour work = (1/12 + 1/20) = 2/15   Part filled in 2 hrs = (3/20 + 2/15) = 17/60   Part filled in 6 hrs = 3 * 17/60 = 17/20   Remaining part = 1 - 17/20 = 3/20   Now, it is the turn of A and B and 3/20 part is filled by A and B in 1 hour.   Total time taken to fill the tank = (6 + 1)= 
<gadget id="calculator">(6 + 1)</gadget>
<output>7</output>
7 hrs.Answer: 7 hrs
<result>C</result>
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 is?