A man buy 10 lemon on 20 and sell 8 lemon on 20 find his profit or loss percent

The problem with this problem is I can't tell how much money he has gained because I don't know how much the lemons are actually worth. If the lemons are worth 20 bucks for 10, then he made 4 bucks, but if they are actually worth 20 bucks for 8, then he made 5 bucks. (i believe everything i said is true but i could be wrong)