You are told Y is the number of large vehicles washed and that there were 50 large vehicles.
Replace Y with 50 in the given equation to solve for x ( the number of small cars.
5x + 10(50) = 800
5x + 500 = 800
Subtract 500 from both sides:
5x = 300
Divide both sides by 5:
x = 300/5
x = 60
There were 60 small cars washed.