In a mathematics test, the average score of the whole class is 91.2. It is known that there are 21 girls with an average score of 92 and boys with an average score of 90.5. How many boys are there in this class?
一次数学测试，全班平均分是91.2分，已知女生有21人，平均每人92分，男生平均每人90.5分，这个班男生有多少人．
<gadget id="calculator">92 - 91.2</gadget>
<output>0.8</output>

<gadget id="calculator">91.2 - 90.5</gadget>
<output>0.7</output>

<gadget id="calculator">21 / 0.7</gadget>
<output>30</output>

<gadget id="calculator">0.8 * 30</gadget>
<output>24</output>

<result>24</result>