Students in two classes of fifth grade collected tree species. Class 51 collected 12.4 kg, which was 2.8 kg less than class 52. How many kg of tree species were collected by the two classes?
五年级两个班的学生采集树种，五1班采集12.4千克，比五2班少采2.8千克，两个班一共采集树种多少千克？
<gadget id="calculator">12.4 + 2.8 + 12.4</gadget>
<output>27.6</output>

<result>27.6</result>