Students in the fourth and fifth grades collected tree species. The fifth grade collected 19.4 kilograms, and the fifth grade collected 3.5 kilograms more than the fourth grade. How many kilograms of tree species were collected in the fourth grade?
四、五年级的学生采集树种，五年级采集了19.4千克，五年级采集的比四年级多3.5千克，四年级采集多少千克树种？
<gadget id="calculator">19.4 - 3.5</gadget>
<output>15.9</output>

<result>15.9</result>