There are 200 trees on the way from Xiao Ming's house to the swimming pool. On the way back and forth, Xiao Ming marked some trees with red ribbons. When he went to the swimming pool, he marked the first tree, the sixth tree, the eleventh tree, ... every time. Mark one of the 4 trees; when returning, he marked the 1st tree, the 9th tree, the 17th tree, ..., every time he marked one tree every 7 trees. Then when he returns home, how many trees are not marked?
从小明家到游泳池的路上有200棵树．在往返的路途中，小明用红丝带系在一些树上做标记，去游泳池的时候，他在第1棵树、第6棵树、第11棵树、…上做了标记，每次都隔4棵树标记一棵；返回时，他在遇到的第1棵树、第9棵树、第17棵树、…上做了标记，每次都隔7棵树标记一棵．则他回到家时，没有被标记的树共有多少棵．
<gadget id="calculator">200 / 5</gadget>
<output>40</output>

<gadget id="calculator">200 / 8</gadget>
<output>25</output>

<gadget id="calculator">5 * 8</gadget>
<output>40</output>

<gadget id="calculator">200 / 40</gadget>
<output>5</output>

<gadget id="calculator">25 - 5</gadget>
<output>20</output>

<gadget id="calculator">40 + 20</gadget>
<output>60</output>

<gadget id="calculator">200 - 60</gadget>
<output>140</output>

<result>140</result>