Farmer Brian has 3 acres of land which he farms efficiently. Each acre can support 10 apple trees. However the 3 acres differ in their ability to support orange trees. He can grow 30 orange trees on the best land, 20 orange trees on the ok land, and 10 orange trees on the bad land. If he initially has all is land growing apples, what would be the opportunity cost of growing an orange tree?

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