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. 20/3hours
C. 7 hours
D. 15/2hours

• Taking the lcm(12,15,20) = 60 liters (capacity of the tank)
  In 1 hr : A can fill 5 liters(60/12), B = 4 liters C = 3 liters
  Its given that A is open all time and B & C alternately.
  So in 1st hr : A+B = 9 liters
  2nd hr : A+C = 8 liters
  3rd hr : A+B = 9 liters
  4th hr : A+C = 8 liters
  5th hr : A+B = 9 liters
  6th hr : A+C = 8 liters
  7th hr : A+B = 9 liters
  Total = 60 liters (Capacity of the tank).
  So Ans = C. 7 hours.
  
  
  
• 9 years agoHelpfull: Yes(11) No(0)
• (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 and 3 part is filled by A and B in 1 hour.
  20
  Total time taken to fill the tank = (6 + 1) hrs = 7 hrs.
• 11 years agoHelpfull: Yes(4) No(11)
• (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 and 3 part is filled by A and B in 1 hour.
  20
  Total time taken to fill the tank = (6 + 1) hrs = 7 hrs.
• 8 years agoHelpfull: Yes(0) No(0)