In ten years, I'll be twice my brother's age. The sum of our ages will then be 45 years old. How old am I now?
Let X be my age now.
In ten years, I'll be X+
<gadget id="calculator">+10</gadget>
<output>10</output>
10 years old.
In ten years, my brother will be (X+10)*1/2 years old.
In ten years, (X+10) + (X+10)*1/2 = 45.
So (X+10)*3/2 = 45.
X+10 = 45 * 2/3 = 30.
X = 30 - 10 = 
<gadget id="calculator">30-10</gadget>
<output>20</output>
20 years old.

<result>20</result>