The fifth grade collected tree species. Class 5 1 collected 20.4 kg, which was 3.9 kg more than Class 5 2. How many kg of tree species were collected by the two classes?
五年级采集树种，五1班采了20.4千克，比五2班多采集3.9千克，两个班一共采集树种多少千克？
<gadget id="calculator">20.4 - 3.9</gadget>
<output>16.5</output>

<gadget id="calculator">16.5 + 20.4</gadget>
<output>36.9</output>

<result>36.9</result>