The average of four numbers is 50. If one of the numbers is rewritten as 60, the average of these four numbers becomes 58. What is the changed number?
四个数的平均数是50，把其中一个数改写成60，这四个数的平均数变成58，被改变的数原来是多少．
<gadget id="calculator">58 - 50</gadget>
<output>8</output>

<gadget id="calculator">8 * 4</gadget>
<output>32</output>

<gadget id="calculator">60 - 32</gadget>
<output>28</output>

<result>28</result>