There are seven numbers arranged in a row, their average is 30, the average of the first three numbers is 28, and the average of the last five numbers is 33. Find the third number.
有七个排成一列的数，它们的平均数是30，前三个数的平均数是28，后五个数的平均数是33．求第三个数．
<gadget id="calculator">28 * 3</gadget>
<output>84</output>

<gadget id="calculator">33 * 5</gadget>
<output>165</output>

<gadget id="calculator">84 + 165</gadget>
<output>249</output>

<gadget id="calculator">30 * 7</gadget>
<output>210</output>

<gadget id="calculator">249 - 210</gadget>
<output>39</output>

<result>39</result>