Two pipes A and B can fill a tank in 12 hours and 10 hours respectively. If both the pipes are opened simultaneously in the empty tank, how much time will taken by them to fill it ?       

The part filled by Pipe A in 1 hour = $\frac{1}{12}$

The part filled by Pipe B in 1 hour = $\frac{1}{10}$

$totaltime=\frac{volume}{volumefilledin1hour}$

Volume  filled by both in 1 hour$A\&B\Rightarrow A+B=\frac{1}{12}+\frac{1}{10}=\frac{5+6}{60}=\frac{11}{60}$

$Totaltime=\frac{1}{\frac{11}{60}}\Rightarrow \frac{60}{11}hrs\Rightarrow 5hr\frac{5}{11}\min$