Three pipes A, B and C can fill a tank in 6 hrs. All the pipes are opened for 2 hrs and then C turned off. A and B filled the remaining in 7hrs. In how many hours pipe C alone can fill it.

$A+B+C fills in 6hours$

In 1 hour $\frac{1}{6} part$ is filled 

If all are open for 2 hours then $\frac{1}{3}$ 

$A\&B$together filled in 7 hour 

let A and B together completes the work in x days

$\frac{1}{x}\times 7=\frac{2}{3}\to x=\frac{21}{2}$

c per hour  can fill the tank  $\frac{1}{6}-\frac{1}{21/2}=\frac{1}{6}-\frac{2}{21}=\frac{7-4}{42}=\frac{3}{42}=\frac{1}{14}$

c alone fills the tank in =14 hours