In a mathematics test, the average score of the whole class is 91.2 points. It is known that there are 21 girls, with an average score of 92 points per person, and an average score of 90.5 points per boy student. How many boys are there in this class?
一次数学测验，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．求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>