If 36 fruits were evenly split between 3 types of fruits, then I bought 36/3 = <<36/3=12>>12 units of each fruit
If 1 orange costs $0.50 then 12 oranges will cost $0.50 * 12 = $<<0.5*12=6>>6
If my total bill was $66 and I spent $6 on oranges then I spent $66 - $6 = $<<66-6=60>>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 = <<60=60>>60A
Then we know the price of one apple (A) is $60/60= $<<60/60=1>>1
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