3 zeus = 1 skeletons
zeus + dragon + 3 = 3*zeus
dragon = 2*zeus - 3
dragon - 2 = zeus
zeus + 2 = 2*zeus - 3
zeus = 5
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skeletons = 3*zeus = 15
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dragon = 2*zeus - 3 = 2*5 - 3 =7
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vessel = dragon*3 = 7*3 = 21
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answer = vessel + skeletons = 21 + 15 = 36