There are seven numbers in a column. The average of these seven numbers is 38, the average of the first four numbers is 31, and the average of the last four numbers is 46. What is the fourth number in this column?
一列数共七个，这七个数的平均数是38，前四个数的平均数是31，后四个数的平均数是46，这列数中第四个数是多少．
<gadget id="calculator">31 * 4</gadget>
<output>124</output>

<gadget id="calculator">46 * 4</gadget>
<output>184</output>

<gadget id="calculator">38 * 7</gadget>
<output>266</output>

<gadget id="calculator">184 - 266</gadget>
<output>-82</output>

<gadget id="calculator">124 + (-82)</gadget>
<output>42</output>

<result>42</result>