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=
<gadget id="calculator">6/2</gadget>
<output>3</output>
3 yellow butterflies.
Martha has 6+3=
<gadget id="calculator">6+3</gadget>
<output>9</output>
9 blue and yellow butterflies.
Martha has 19-9=
<gadget id="calculator">19-9</gadget>
<output>10</output>
10 black butterflies.

<result>10</result>