The average of 4 numbers is 30, if one of them is changed to 50, the average is 40, what is the original number?
某4个数的平均值是30，把其中一个改为50，平均值为40，这个数原来是多少？
<gadget id="calculator">40 * 4</gadget>
<output>160</output>

<gadget id="calculator">30 * 4</gadget>
<output>120</output>

<gadget id="calculator">160 - 120</gadget>
<output>40</output>

<gadget id="calculator">50 - 40</gadget>
<output>10</output>

<result>10</result>