Algebra 1
posted by NEED HELP NOW!! .
The first of three pipes fills a tank in 12 minutes and a second pipe in 10 minutes. If both these pipes and also a third pipe are open the tank is filled in 4 min. How many minutes will the third pipe alone require to fill the tank?
My teacher told me to use a chart and an equation for this question. so any help i can get would be great. I'm sorta clueless of how to label the chart, also! thanks!

just add up the amount each pipe does in one minute. The result must equal how much is done by everyone in 1 minute:
1/x + 1/12 + 1/10 = 1/4
x = 15