To move 18 tables. Each adult carries one, and the two children carry one. Now there are 3 adults, how many children can be moved at once?
要搬18张桌子．每个大人搬一张，两小朋友抬一张．现在有3个大人，还要多少个小朋友才能一次搬完？
<gadget id="calculator">18 - 3</gadget>
<output>15</output>

<gadget id="calculator">15 * 2</gadget>
<output>30</output>

<result>30</result>