There are 40 people in class one of the third grade, 18 people signed up for the calligraphy club, 20 people signed up for the chorus club, and 15 people didn't sign up for these two clubs. How many people signed up for both the calligraphy club and the chorus club?
三年级一班有40人，报名参加书法社团的有18人，报名参加合唱社团的有20人，没报名参加这两个社团的有15人．既报名参加书法社团又报名参加合唱社团的有多少人？
<gadget id="calculator">18 + 20</gadget>
<output>38</output>

<gadget id="calculator">40 - 15</gadget>
<output>25</output>

<gadget id="calculator">38 - 25</gadget>
<output>13</output>

<result>13</result>