It takes 34 minutes to walk back and forth between A and B; it takes 21 minutes to go there by bicycle and then walk back. How many minutes does it take to go back and forth by bicycle?
甲、乙两地步行往返需34分钟；骑自行车去，然后步行回来需21分钟，骑自行车往返需多少分钟．
<gadget id="calculator">34 / 2</gadget>
<output>17</output>

<gadget id="calculator">21 - 17</gadget>
<output>4</output>

<result>4</result>