To make orange paint, pedro mixes red paint and yellow paint in the ratio 3: 2. if he uses 8.5 gallons of yellow paint, how many gallons of red paint does he use?

You can set up the equation 3y=2r, with r standing for the amount of red paint in gallons, the same going for y.  If you substitute in 8.5 for y, you get that 2r=25.5.  Divie each side by 2, and r=12.75.  So, Pedro uses 12.75 (or 12 and three-fourths) gallons of red paint.