The students have summer vacation on July 15th, and come to school on September 1st. How many days are there in the summer vacation?
学生7月15日放暑假，9月1日到校上课，暑假共多少天．
<gadget id="calculator">31 - 15</gadget>
<output>16</output>

<gadget id="calculator">16 + 1</gadget>
<output>17</output>

<gadget id="calculator">31 + 17</gadget>
<output>48</output>

<result>48</result>