In a campus beautification activity, the school first arranged 31 people to pull weeds, 18 people to plant trees, and then sent 20 people to support them. As a result, the number of people who pulled weeds was twice as many as those who planted trees. how many people?
在一次美化校园的活动中，学校先安排31人去拔草，18人去植树，后增派20人去支援他们，结果拔草人数是植树人数的2倍，问去支援拔草的人数有多少人？
<gadget id="calculator">2 * 18</gadget>
<output>36</output>

<gadget id="calculator">2 * 20</gadget>
<output>40</output>

<gadget id="calculator">40 - 31</gadget>
<output>9</output>

<gadget id="calculator">36 + 9</gadget>
<output>45</output>

<gadget id="calculator">1 + 2</gadget>
<output>3</output>

<gadget id="calculator">45 / 3</gadget>
<output>15</output>

<result>15</result>