Uncle Li was walking on the road, and a row of trees was evenly planted on the side of the road. It took 7 minutes for Uncle Li to walk from the first tree to the fifteenth tree. Uncle Li walked forward a few more trees and then turned back. Walking, it took 30 minutes to get back to the fifth tree. When Uncle Li walked to the first tree, he started to walk back?
李大爷在马路上散步，路边均匀地栽着一行树，李大爷从第一棵树走到第十五棵树，用了7分钟，李大爷又往前走了几棵树后就往回走，当回到第五棵树时共用了30分钟．李大爷散步到第几棵树时开始往回走？
<gadget id="calculator">15 - 1</gadget>
<output>14</output>

<gadget id="calculator">7 / 14</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">30 / (1/2)</gadget>
<output>60</output>

<gadget id="calculator">5 - 1</gadget>
<output>4</output>

<gadget id="calculator">60 - 4</gadget>
<output>56</output>

<gadget id="calculator">56 / 2</gadget>
<output>28</output>

<gadget id="calculator">28 + 4 + 1</gadget>
<output>33</output>

<result>33</result>