In the math class, Mrs. Ye assigned two math problems. As a result, 15 people got the first problem right; 24 people got the second problem right; All are 4 people. So, how many people are there in this class?
数学课上，叶老师布置了两道数学题，结果有15人做对了第一道题；有24人做对了第二道题；两道题都做对的与两道题都做错的都是4人．那么，这个班上共有多少人．
<gadget id="calculator">4 - 4</gadget>
<output>0</output>

<gadget id="calculator">15 + 24 + 0</gadget>
<output>39</output>

<result>39</result>