If I'm 4 times older than Billy currently, and Billy is 4 years old, how old was I when Billy was born?
I would currently be 4*4=
<gadget id="calculator">4*4</gadget>
<output>16</output>
16 years old, since I'm 4 times older than Billy's age of 4.
Therefore, in order to find my age at Billy's birth we'd simply subtract Billy's age of 4 from my age, meaning I was 16-4= 
<gadget id="calculator">16-4</gadget>
<output>12</output>
12 years old.

<result>12</result>