The voyage between port A and port B is 500 kilometers. A cargo ship sails from port A to port B at 10 am (downstream), and a passenger ship departs from port B to port A at 2 pm. The passenger ship sailed 12 hours to meet the cargo ship. It is known that the cargo ship travels 15 kilometers per hour and the water velocity is 5 kilometers per hour. How many kilometers per hour does the passenger ship travel?
甲、乙两港的航程有500千米，上午10点一艘货船从甲港开往乙港(顺流而下)，下午2点一艘客船从乙港开往甲港．客船开出12小时与货船相遇．已知货船每小时行15千米，水流速度每小时5千米，客船每小时行多少千米？
<gadget id="calculator">15 + 5</gadget>
<output>20</output>

<gadget id="calculator">20 * 4</gadget>
<output>80</output>

<gadget id="calculator">500 - 80</gadget>
<output>420</output>

<gadget id="calculator">420 / 12</gadget>
<output>35</output>

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

<result>20</result>