You ran out of paint and primer while changing the color tone in your new house. You had already purchased 4 gallons of paint and 9 gallons of primer for a total cost of $275. Back to the paint shop, added 3 more gallons of paint and 5 gallons of primer for $180. Find the cost per gallon for the paint and the pine.

let cost of paint be x and cost of primer be y 4x + 9y = 275 3x + 5y = 180 muliply the 1st equation with -3 and the 2nd equation by 4 -12x - 27y = -825 12x + 20y = 720 add both equations -7y = -105 y = 15 substitute to equation 1 4x + 9*15 = 275 4x = 275 - 135 4x = 140 x = 35 therefore cost per gallon of paint is 35 while cost per gallon of primer is 15.