Wang Lei was born on National Day, if 3 times his age minus 8 is just the total number of days in the month he was born, then how old is Wang Lei this year?
王雷是国庆节那天出生的，若他年龄的3倍减去8刚好是他出生那月的总天数，则王雷今年多少岁．
<gadget id="calculator">31 + 8</gadget>
<output>39</output>

<gadget id="calculator">39 / 3</gadget>
<output>13</output>

<result>13</result>