There are 52 students in a class, 48 of whom can play chess, 37 can draw, and 39 can dance. How many people in this class can do all three.
某班有52人，其中会下棋的有48人，会画画的有37人，会跳舞的有39人，这个班三项都会的至少有多少人．
<gadget id="calculator">52 - 48</gadget>
<output>4</output>

<gadget id="calculator">52 - 37</gadget>
<output>15</output>

<gadget id="calculator">52 - 39</gadget>
<output>13</output>

<gadget id="calculator">4 + 15 + 13</gadget>
<output>32</output>

<gadget id="calculator">52 - 32</gadget>
<output>20</output>

<result>20</result>