To paint his house, Samuel bought 2 cans of paint and one roller for $30. When friends came over to help, he went back to the store and bought three more cans of paint and two more rollers for $50. How much did he pay for a can of paint?

I was trying to recall (if I ever knew) how to set that up in an equation, since you are not supposed to solve by trial and error. I set out using x as the price of paint and y as the price of a roller, though I knew somehow you are supposed to reduce to one unknown variable. All at once it became apparent that both scenarios work if both the paint and the rollers are priced at $10 each.