The fourth-grade students collected 30.7 kg of scrap steel, and the third-grade students collected 4.2 kg less than the fourth-grade students. How many kilograms did the two grades collect in total?
四年级学生收集废钢铁30.7千克，三年级比四年级少收集4.2千克．两个年级共收集多少千克？
<gadget id="calculator">30.7 - 4.2</gadget>
<output>26.5</output>

<gadget id="calculator">26.5 + 30.7</gadget>
<output>57.2</output>

<result>57.2</result>