There are red, green and yellow balls in a cardboard box, the number of red balls is 3/4 of the number of green balls, and the ratio of the number of green balls to yellow balls is 4:5. Given that there are 81 green balls and yellow balls, how many are there yellow balls?
纸箱里有红、绿、黄三种颜色的球，其中红球的个数是绿球的(3/4)，绿球与黄球个数的比是4：5．已知绿球和黄球共有81个，则黄球有多少个．
<gadget id="calculator">4 + 5</gadget>
<output>9</output>

<gadget id="calculator">81 / 9</gadget>
<output>9</output>

<gadget id="calculator">9 * 5</gadget>
<output>45</output>

<result>45</result>