There are 40 pupils in a class. Half of the class likes blue. One-fourth of the remaining likes green and the rest likes yellow. How many pupils like the color yellow?
There are 40/2 = 
<gadget id="calculator">40/2</gadget>
<output>20</output>
20 pupils who like blue.
So, there are now 40 - 20 = 
<gadget id="calculator">40-20</gadget>
<output>20</output>
20 pupils who like either green or yellow.
Hence, 20/4 = 
<gadget id="calculator">20/4</gadget>
<output>5</output>
5 pupils like green.
Thus, 20 - 5 = 
<gadget id="calculator">20-5</gadget>
<output>15</output>
15 pupils like yellow.

<result>15</result>