Martha has 19 butterflies in her collection. She has twice as many blue butterflies as yellow butterflies. The rest of her butterflies are black. If Martha has 6 blue butterflies, how many black butterflies does she have? Think carefully first, then make a decision:
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Martha has 6 / 2 = 3 yellow butterflies. Martha has 6 + 3 = 9 blue and yellow butterflies. Martha has 19 - 9 = 10 black butterflies. So, the answer is 10.
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