There are seven numbers, the average number is 49, the average number of the first 4 numbers is 28, and the average number of the last 4 numbers is 68.25, then what is the fourth number?
有七个数，平均数为49，前4个数的平均数为28，后4个数的平均数为68.25，那么第4个数为多少．
<gadget id="calculator">28 * 4</gadget>
<output>112</output>

<gadget id="calculator">68.25 * 4</gadget>
<output>273</output>

<gadget id="calculator">49 * 7</gadget>
<output>343</output>

<gadget id="calculator">273 - 343</gadget>
<output>-70</output>

<gadget id="calculator">112 + (-70)</gadget>
<output>42</output>

<result>42</result>