You can buy 4 apples or 1 watermelon for the same price. You bought 36 fruits evenly split between oranges, apples and watermelons, and the price of 1 orange is $0.50. How much does 1 apple cost if your total bill was $66? Think carefully first, then make a decision:
If 36 fruits were evenly split between 3 types of fruits, then I bought 36 / 3 = 12 units of each fruit. If 1 orange costs $0.50 then 12 oranges will cost $0.50 * 12 = $6. If my total bill was $66 and I spent $6 on oranges then I spent $66 - $6 = $60 on the other 2 fruit types. Assuming the price of watermelon is W, and knowing that you can buy 4 apples for the same price and that the price of one apple is A, then 1W = 4A. If we know we bought 12 watermelons and 12 apples for $60, then we know that $60 = 12W + 12A. Knowing that 1W = 4A, then we can convert the above to $60 = 12(4A) + 12A. $60 = 48A + 12A. $60 = 60A. Then we know the price of one apple (A) is $60 / 60 = $1. So, the answer is 1.