If one day of a month is the 16th day counting from the 1st, and counting from the end of the month is also 16 days, how many days are there in this month?
如果某月的一天从1号数起是第16天，从月底倒数起也是16天，这个月共有多少天．
<gadget id="calculator">15 + 15 + 1</gadget>
<output>31</output>

<result>31</result>