Two pipes A and B can fill a tank in 12 minutes and 18 minutes respectively. If both the pipes are opened simultaneously, how long will they take to fill the tank ?

Explanation

The part of the tank filled by A in 1 minute `=(1)/(12)`
The part of the tank filled by B in 1 minute `=(1)/(18)`
The part of the tank filled by both the pipes in 1 minute `=(1)/(12)+(1)/(18)=(5)/(36)`
`:.` The tank can be fillled in `(36)/(5)=7(1)/(5)` minutes