well she automatically makes 40,000 a year. lets say x is the amount of cars she sells. and she gets 1,000 for each sold. so if she sells 2 cars she would get 2 * 1,000. 3 cars would be 3 * 1,000. but we just want to say x cars so we use x * 1,000.
so she makes 40,000 plus her commission which is x * 1,000. so if the amount she makes is represented by y then:
y = 40,000 + (x * 1,000)
if she wants to make 60,000 in a year then y = 60,000. substitute that into our equation and get
60,000 = 40,000 + (x * 1,000)
20,000 = x * 1,000
x = 20.
so she would have to sell 20 cars