According to the ratio of the number of girls to the number of boys (9/10), make up two problems calculated by different methods and solve them.
根据女生人数占男生人数的(9/10)，编两道用不同方法计算的应用题并解答出来．
<gadget id="calculator">9 / 10</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">20 * (9/10)</gadget>
<output>18</output>

<gadget id="calculator">18 / (9/10)</gadget>
<output>20</output>

<gadget id="calculator">18 + 20</gadget>
<output>38</output>

<result>38</result>