I suck at word problems. Anyhow, can anyone help me solve/ set this problem up???

#) An oil tank at Chemco Refinery has two inlet pipes and one outlet pipe. One inlet pipe can fill the tank in 6 hours, and the other inlet pipe can fill the tank in 12 hours. The outlet pipe can EMPTY the tank in 24 hours. How long would it take to fill the tank with all three pipes open?

thx