There are 80 young people, each of whom can either ride a bicycle or swim, or both, now we know that there are 65 people who can ride a bicycle, 30 people who can ride a bicycle and swim, how many people can swim.
有80个青年，其中每个青年或者会骑自行车或者会游泳，或者两项都会，现在知道会骑自行车的有65人，会骑自行车又会游泳的有30人，会游泳的有多少人．
<gadget id="calculator">30 - 65</gadget>
<output>-35</output>

<gadget id="calculator">80 + (-35)</gadget>
<output>45</output>

<result>45</result>