There are two wharves A and B on the river, 30 kilometers apart. Wharf A is upstream of Wharf B. A cargo ship and a cruise ship start from both Wharf A and B and head downstream at the same time. After 2 hours, the cargo ship catches up with the cruise ship. After driving for another hour, an object on the cargo ship fell into the river (the object could float on the water surface). After 10 minutes, the people on the cargo ship found it and turned around to look for it. When they found it, they happened to meet the cruise ship again. So, what is the speed of the cruise ship in still water?
江上有A、B两座码头，相距30千米，A码头在B码头的上游，一艘货船和一艘游船同时从A码头和B码头出发向下游行驶，2小时后货船追上游船．又行驶了1小时，货船上有一物品落入江中(该物品可以浮在水面上)，10分钟后被货船上的人发现了，便掉转船头去找，找到时恰好又和游船相遇．那么，游船在静水中的速度为多少？
<gadget id="calculator">30 / 2</gadget>
<output>15</output>

<gadget id="calculator">15 * 1</gadget>
<output>15</output>

<gadget id="calculator">1 / 6</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">15 * 1 * (1/6)</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">15 + (5/2)</gadget>
<output>35/2 = around 17.5</output>

<gadget id="calculator">(35/2) / (1/6)</gadget>
<output>105</output>

<gadget id="calculator">105 - 15</gadget>
<output>90</output>

<gadget id="calculator">90 / 2</gadget>
<output>45</output>

<result>45</result>