They wash a car for $5, so 5 represents the cost of washing one car. to find out how much they will earn by washing x cars, we can multiply 5 by any number to find the total cost of the washed cars. For example 5×2=10 which means the cost of washing 2 cars is $10 and 5×3=15 means the cost of washing 3 cars is $15 and also 5×4=20 means the cost of washing 4 cars is $20. Basically the cost of the washed cars will increase by $5 each time the number of cars increase.