First you need yo figure out how much each gallon of paints costs. so divide 55 by 10. 55/10=5.5. So each gallon costs $5.50. After that you divide 30 by 5.5 to get the amount of paint you can buy for $30.00. 30/5.5=5.45454545..round that to get 5) so you can buy 5 gallons of paint for $30
