There are six numbers lined up, their average is 65, the average of the first four numbers is 55, the average of the last three numbers is 80, what is the fourth number.
有六个数排成一行，它们的平均数是65，前四个数的平均数是55，后三个数的平均数是80，第四个数是多少．
<gadget id="calculator">55 * 4</gadget>
<output>220</output>

<gadget id="calculator">80 * 3</gadget>
<output>240</output>

<gadget id="calculator">65 * 6</gadget>
<output>390</output>

<gadget id="calculator">240 - 390</gadget>
<output>-150</output>

<gadget id="calculator">220 + (-150)</gadget>
<output>70</output>

<result>70</result>