A school purchased a total of 47 projectors of three types: large, medium, and small. Their unit prices were 700 yuan, 300 yuan, and 200 yuan. The school paid a total of 21,200 yuan. It is known that the number of medium-sized projectors is small projectors 2 times the number, how many medium-sized projectors did you buy?
某校购买了大、中、小三种型号的投影仪共47台，它们的单价分别是700元、300元和200元，该校共付款21200元，已知中型投影仪的台数为小型投影仪台数的2倍，那么中型投影仪买了多少台？
<gadget id="calculator">700 * 47</gadget>
<output>32_900</output>

<gadget id="calculator">32_900 - 21_200</gadget>
<output>11_700</output>

<gadget id="calculator">700 * 3</gadget>
<output>2_100</output>

<gadget id="calculator">2 * 300</gadget>
<output>600</output>

<gadget id="calculator">2_100 - 200 - 600</gadget>
<output>1_300</output>

<gadget id="calculator">11_700 / 1_300</gadget>
<output>9</output>

<gadget id="calculator">2 * 9</gadget>
<output>18</output>

<result>18</result>