The average of five numbers is 60. If one of the numbers is changed to 80, the average becomes 70. What is the original number?
五个数的平均数是60，若把其中一个数改为80，平均数变为70，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>