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[Fruit Pie Jones]

Ignore the red and blue triangles, and it makes perfect sense. The green and yellow shapes are arranged in the top image to form a 3 x 5 rectangle, the area of which is, of course, 15 square units. This is easily verified by counting squares if you wish. In the bottom image, those two shapes are arranged to form a near-rectangle with dimensions 8 x 2. The area of that rectangle would be 16 square units, but we already know that the two shapes have a total area of only 15. Thus the empty square.