Three pipes A, B and C can fill a tank 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 tank is :

21 hours

28 hours

35 hours

none

Solution: Part filled in 4 hours = 4/12 = 1/3, remaining part = (1 - 1/3) = 2/3. .'. (A+B)'s 14 hour's work = 2/3; A+B 's 1 hour's work = 2/(3*14). .'. C's 1 hour's work = [(A+B+C)'s 1 hour's work - (A+B)'s 1 hour's work] = (1/12 - 2/42) = (7-4)/84 = 3/84 = 1/28. .'. 'C' alone can fill the tank in 28 hours.

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