The average of 4 numbers is 181. If the fifth number is added, the average becomes 169. What is the fifth number?
4个数的平均数是181，如果加上第五个数，平均数就变成了169，第五个数是多少．
<gadget id="calculator">169 * 5</gadget>
<output>845</output>

<gadget id="calculator">181 * 4</gadget>
<output>724</output>

<gadget id="calculator">845 - 724</gadget>
<output>121</output>

<result>121</result>