Beibei walks home from school, and then rides a bicycle back to school for 13 minutes. If it takes 8 minutes to go to and from both places by bicycle, how many minutes will it take to walk to and from both places?
贝贝步行从学校回到家，然后骑自行车又回到学校共用13分钟，如果往返两地都是骑自行车共用8分钟，问如果往返两地都步行需要多少分钟．
<gadget id="calculator">8 / 2</gadget>
<output>4</output>

<gadget id="calculator">13 - 4</gadget>
<output>9</output>

<gadget id="calculator">9 * 2</gadget>
<output>18</output>

<result>18</result>