John is half times younger than his father, who is 4 years older than John's mother. If John's father is 40 years old, what's the age difference between John and his mother?
John is half times his father's age. Thus 1/2*40 = 
<gadget id="calculator">1/2*40</gadget>
<output>20</output>
20 years.
Since the mother is 4 years younger than the father, she is 40-4= 
<gadget id="calculator">40-4</gadget>
<output>36</output>
36 years old.
The age difference between John and his mother is 36-20 = 
<gadget id="calculator">36-20</gadget>
<output>16</output>
16 years.

<result>16</result>