arithmetic - three pipes a b and c can fill a tank in 6 hours after working at it together for 2 hours c is closed and a and b can fill the remaining part in 7 hours the number of hours taken by c alone to fill the tank is - skillgun

Part filled in 2 hours = 2/6 = 1/3
Remaining part = 1 - 1/3 = 2/3
(A + B)'s 7 hour's work = 2/3
(A + B)'s 1 hour's work = 2/21
C's 1 hour's work = { (A + B + C)'s 1 hour's work } - { (A + B)'s 1 hour's work }
= 1/6 - 2/21 = 1/14
C alone can fill the tank in 14 hours.