You and your friend each start a car-washing service. you spend $25 on supplies and charge $10 per car. your friend spends $55 on supplies and charges $13 per car. how many cars do you have to wash to earn the same amount of money as your friend?

Let x be the number of cars. We will find out how many cars you have to wash to make as much money as your friend:
10x - 25= 13x - 55 Add 25 on both sides and subtract 13x, also both sides:
3x = -30 Now divide both sides by -3:
x= 10 cars you have to wash.