There are 4 10x15 ft walls. A walls measures are 10x15 ft and its area is 150 ft². 4 walls will make 150x4 = 600 ft². As he won't paint the 3x5 ft (15 ft²) window and the 3.5x7 ft (24.5 ft²) door, we can subtract them from the total area of walls.

600 - 15 = 585

585 - 24.5 = 560.5 ft²

If 1 gallon can of paint can cover 300-350 ft², and the question asks for the best situation, we will take it as 350 ft². So he will need 2 cans to paint 560.5 ft².

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