John is 10 years old. His sister is twice his age. When he is 50 years old, how old will his sister be?
His sister is 10*2= 
<gadget id="calculator">10*2</gadget>
<output>20</output>
20 years old.
Their age difference is 20-10= 
<gadget id="calculator">20-10</gadget>
<output>10</output>
10 years.
Therefore when he is 50 she will be 50+10= 
<gadget id="calculator">50+10</gadget>
<output>60</output>
60 years old.

<result>60</result>