Lee washes the outsides of houses. It takes him 40 minutes to power wash a one-story home, and he uses 180 gallons of water. Power washing a two-story home takes 90 minutes and uses 300 gallons of water. Lee works no more than 40 hours each week, and he uses a maximum of 5,000 gallons of water per week. He charges $90 to wash a one-story home and $150 to wash a two-story home. Lee wants to maximize his weekly washing income washing houses. Let x represent the number of one-story houses and y represent the number of two-story houses. What are the constraints for the problem?