Marianne was 20 years old when Bella was 8 years old. How old will Marianne be when Bella turns 18?
Marianne is 20 - 8 = 
<gadget id="calculator">20-8</gadget>
<output>12</output>
12 years older than Bella.
Bella will turn 18 in 18 - 8 = 
<gadget id="calculator">18-8</gadget>
<output>10</output>
10 years.
So, Marianne will be 20 + 10 = 
<gadget id="calculator">20+10</gadget>
<output>30</output>
30 years old then.

<result>30</result>