Uncle Wang is walking on the side of the road. There is a row of trees evenly growing on the side of the road. It took 420 seconds for Uncle Wang to walk from the first tree to the 15th tree. When he walked back to the fifth tree, it took 1800 seconds. Uncle Wang walked to the first tree and started to walk back.
王大爷在马路边散步，路边均匀地长着一行树，王大爷从第1棵走到第15棵树用了420秒，王大爷又往前走若干棵树后就往回走．当他走回第5棵树时，共用了1800秒．王大爷散步到第多少棵树就开始往回走．
<gadget id="calculator">1_800 - 420</gadget>
<output>1_380</output>

<gadget id="calculator">15 - 1</gadget>
<output>14</output>

<gadget id="calculator">420 / 14</gadget>
<output>30</output>

<gadget id="calculator">1_380 / 30</gadget>
<output>46</output>

<gadget id="calculator">15 - 5</gadget>
<output>10</output>

<gadget id="calculator">46 - 10</gadget>
<output>36</output>

<gadget id="calculator">36 / 2</gadget>
<output>18</output>

<gadget id="calculator">15 + 18</gadget>
<output>33</output>

<result>33</result>