There are five numbers in a row, their average is 54, the average of the first three numbers is 49, and the average of the last three numbers is 51, find the third number.
有五个数排成一排，它们的平均数是54，前三个数的平均数是49，后三个数的平均数是51，求第三个数．
<gadget id="calculator">49 * 3</gadget>
<output>147</output>

<gadget id="calculator">51 * 3</gadget>
<output>153</output>

<gadget id="calculator">54 * 5</gadget>
<output>270</output>

<gadget id="calculator">153 - 270</gadget>
<output>-117</output>

<gadget id="calculator">147 + (-117)</gadget>
<output>30</output>

<result>30</result>