The average of 5 numbers is 60. If one of the numbers is changed to 80, the average becomes 70. What is the changed number?
某5个数的平均数为60，如果把其中一个数改为80，则平均数变为70，改动的这个数原来是多少？
<gadget id="calculator">70 * 5</gadget>
<output>350</output>

<gadget id="calculator">60 * 5</gadget>
<output>300</output>

<gadget id="calculator">350 - 300</gadget>
<output>50</output>

<gadget id="calculator">80 - 50</gadget>
<output>30</output>

<result>30</result>