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?
Martha has 6/2=<<6/2=3>>3 yellow butterflies.
Martha has 6+3=<<6+3=9>>9 blue and yellow butterflies.
Martha has 19-9=<<19-9=10>>10 black butterflies.
#### 10