There are 37 people in class 31, among them 28 people can ride a bicycle, 25 people can skate, 4 people can't do both sports, how many people can do both sports.
三1班有37人，其中会骑车的有28人，会滑冰的有25人，两项运动都不会的有4人，两项运动都会的有多少人．
<gadget id="calculator">28 + 25</gadget>
<output>53</output>

<gadget id="calculator">37 - 4</gadget>
<output>33</output>

<gadget id="calculator">53 - 33</gadget>
<output>20</output>

<result>20</result>