Each class in a school has 20 students. There are 3 classes. The school as a whole is 50% boys and 50% girls. The first class has 15 girls. The second class has 12 girls. How many boys are in the third class?
The school has 60 students because 3 x 20 = 
<gadget id="calculator">3*20</gadget>
<output>60</output>
60.
The school has 30 girls because 60 x .5 = 
<gadget id="calculator">60*.5</gadget>
<output>30</output>
30.
The third class has 3 girls because 30 - 15 - 12 = 
<gadget id="calculator">30-15-12</gadget>
<output>3</output>
3.
The third class has 17 boys because 20 - 3 = 
<gadget id="calculator">20-3</gadget>
<output>17</output>
17.

<result>17</result>