21 students participated in the tree planting activity and planted 33 trees in total. The number of trees planted by each person is 1, 2, and 3 respectively. It is known that the number of people who planted 1 tree is twice the number of people who planted 2 and 3 trees, how many people planted 3 trees?
21名同学参加植树活动共植树33棵．每人植的棵数分别是1棵、2棵、3棵．已知种1棵的人数是种2棵和3棵人数的2倍，种3棵的有多少人．
<gadget id="calculator">2 + 1</gadget>
<output>3</output>

<gadget id="calculator">21 / 3</gadget>
<output>7</output>

<gadget id="calculator">7 * 2 * 1</gadget>
<output>14</output>

<gadget id="calculator">21 - 14</gadget>
<output>7</output>

<gadget id="calculator">7 * 2</gadget>
<output>14</output>

<gadget id="calculator">33 - 14 - 14</gadget>
<output>5</output>

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

<gadget id="calculator">5 / 1</gadget>
<output>5</output>

<result>5</result>