Three pipes A, B and C can fill a cistern in 12 hours. After working at it together for 4 hours, C is closed and A and B can fill the remaining part in 14 hours. The number of hours taken by C alone to fill the cistern is:
Explanation:
A, B and C together can fill 1/12th of cistern in one hour.
A, B and C together worked for two hours = 4 × (1/12) = 1/3 is filled.
A and B together can fill remaining 2/3 in 14 hours
⇒ A and B together can fill complete cistern in 14 × (3/2) = 21 hours.
A and B can together fill 1/21 of tank in one hour where A, B and C can fill 1/12th in one hour.
⇒ C alone can (1/12) - (1/21) of the cistern in one hour = 1/28th of cistern.
So, C alone can fill the cistern in 28 hours.
Hence, option (b).
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