The ratio of the number of boys to the number of girls in a class is 4:5, if there are 30 girls in a class, how many boys are there?
某班男生人数与女生人数的比是4：5，已知女生有30人，则男生有多少人．
<gadget id="calculator">30 / 5</gadget>
<output>6</output>

<gadget id="calculator">6 * 4</gadget>
<output>24</output>

<result>24</result>