A total of 120 fourth-grade and second-grade students go to plant trees together. One fourth-grade student carries two buckets of water, and two second-grade students carry one bucket of water. The two grades carry a total of 180 buckets of water. How many people are there in the second grade?
四年级和二年级学生共有120人，一起去植树，四年级学生一人提两桶水，二年级学生两人抬一桶水，两个年级共提水180桶，则二年级有多少人．
<gadget id="calculator">120 * 2</gadget>
<output>240</output>

<gadget id="calculator">240 - 180</gadget>
<output>60</output>

<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">2 - (1/2)</gadget>
<output>3/2 = around 1.5</output>

<gadget id="calculator">60 / (3/2)</gadget>
<output>40</output>

<result>40</result>