The distance between A and B ports is 180 kilometers. A passenger ship departs from A port to B port and travels 20 kilometers per hour. After 2 hours, a cargo ship departs from B port and travels to A at a speed of 15 kilometers per hour. How many hours does the cargo ship meet the passenger ship on the way?
甲乙两个港口相距180千米，一艘客船从甲港出发向乙港驶去，每小时行20千米，2小时后一艘货船从乙港出发，以每小时15千米的速度向甲港驶去，货船行驶几小时后在途中与客船相遇？
<gadget id="calculator">20 * 2</gadget>
<output>40</output>

<gadget id="calculator">180 - 40</gadget>
<output>140</output>

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

<gadget id="calculator">140 / 35</gadget>
<output>4</output>

<result>4</result>