One pipe can fill a tank in 36 minutes, a second can fill it in 9 minutes, and a third can fill it in 12 minutes. if the tank is empty, how long will the three pipes, operating together, take to fill it?

The filling rates of each pipe are 1/36, 1/9 and 1/12 respectively.  So if all pipes operated together to fill the tank:

t/36+t/9+t/12=1  making all terms have a common denominator of 36...

(t+4t+3t)/36=36/36

8t=36

t=4.5 minutes  (4 min 30 sec)

So it would take all three pipes four and a half minutes to fill the tank.