Pipe and tank

Three pipes A,B,C can fill a tank in 6 hrs.After working together for 2 hrs, C is closed and A & B can fill the remaining in 7 hrs.The number of hrs taken by C alone to fill the tank is
a.10
b.12
c.14
d.16

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1/a + 1/b +1/c = 1/6..now in 2 hrs they'l cmplt 1/3 wrk.so the remaining 2/3 wrk is dne in 7 days.so whole wrk by a and b is done in 10.5 days.so 1/a + 1/b=2/21..by solvn 2 eqn we get c=14.