In March, spring is here. The students went to the spring outing together. There were 27 people who brought biscuits, 21 people who brought bread, and 18 people who brought both biscuits and bread. Everyone brought at least one kind. How many people participated in the spring outing? people.
三月春天来了，同学们集体去春游，带饼干的有27人，带面包的有21人，既带饼干又带面包的有18人，每人至少带一种，参加春游的一共有多少人．
<gadget id="calculator">27 - 18</gadget>
<output>9</output>

<gadget id="calculator">21 + 9</gadget>
<output>30</output>

<result>30</result>