There are two cities along the river with a distance of 600 meters. It takes 35 hours for boat A to go back and forth between the two cities. It takes 5 hours less to travel along the current than against the current. The speed of boat B in still water is 15 kilometers per hour. How many hours does it take to go back and forth between the two cities?
沿江有两个城市，相距600米，甲船往返两城市需35小时，其中顺水比逆水少用5小时，乙船在静水中的速度是每小时15千米，那么往返两城市需要多少小时？
<gadget id="calculator">35 - 5</gadget>
<output>30</output>

<gadget id="calculator">30 / 2</gadget>
<output>15</output>

<gadget id="calculator">600 / 15</gadget>
<output>40</output>

<gadget id="calculator">35 - 15</gadget>
<output>20</output>

<gadget id="calculator">600 / 20</gadget>
<output>30</output>

<gadget id="calculator">40 - 30</gadget>
<output>10</output>

<gadget id="calculator">10 / 2</gadget>
<output>5</output>

<gadget id="calculator">15 + 5</gadget>
<output>20</output>

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

<gadget id="calculator">600 / 10</gadget>
<output>60</output>

<gadget id="calculator">30 + 60</gadget>
<output>90</output>

<result>90</result>