Eustace is twice as old as Milford. In 3 years, he will be 39. How old will Milford be?
Eustace's current age must be 39 years old - 3 years = 
<gadget id="calculator">39-3</gadget>
<output>36</output>
36 years old.
So Milford's current age must be 36 years old / 2 = 
<gadget id="calculator">36/2</gadget>
<output>18</output>
18 years old.
So in 3 years, Milford will be 18 years old + 3 years = 
<gadget id="calculator">18+3</gadget>
<output>21</output>
21 years old.

<result>21</result>