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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 ,C are open for one hour each alternatively, the tank will be full in:

Correct Answer: 7 hrs

Explanation:

A + B ' s   1   h o u r   w o r k = 1 12 + 1 15 = 9 60 = 3 20


A + C ' s   1   h o u r   w o r k = 1 12 + 1 20 = 8 60 = 2 15   


 


p a r t   f i l l e d   i n   2   h r s = 3 20 + 2 15 = 17 60   


 


p a r t   f i l l e d   i n   6   h r s = 3 * 17 60 = 17 20   


 


  r e m a i n i n g   p a r t   = 1 - 17 20 = 3 20   


 


Now, it is  the turn of A and B (3/20) part is filled by A and B in 1 hour.


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


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