A total of 120 sixth-grade students and first-grade students watered the tree together. One sixth-grade student carried two buckets of water, two first-grade students carried one bucket of water, and two grades watered 180 buckets at a time. Ask how many first-grade students there are ?
六年级学生和一年级学生共120人一起给树浇水，六年级学生一人提两桶水，一年级学生两人抬一桶水，两个年级一次浇水180桶，问有一年级学生多少人？
<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>