It takes 30 minutes to ride a bicycle from place A to place B, and it takes 30 minutes to walk back, and it only takes 18 minutes to ride a bicycle for the round trip, so how many minutes will it take to walk both ways.
从甲地到乙地骑车去，步行返回共用30分钟，往返都骑车只要18分钟，那么往返都步行要多少分钟．
<gadget id="calculator">18 / 2</gadget>
<output>9</output>

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

<gadget id="calculator">21 * 2</gadget>
<output>42</output>

<result>42</result>