Two trains depart from A and B at the same time. It takes 20 hours for the express train to complete the journey, and 30 hours for the slow train to complete the journey. 15 hours after leaving, the two cars met. It is known that the express train stops for 4 hours, how many hours does the slow train stop?
两列火车同时从甲、乙两地相对开出．快车行完全程需要20小时，慢车行完全程需要30小时．开出后15小时两车相遇．已知快车中途停留4小时，慢车停了几小时？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">15 - 4</gadget>
<output>11</output>

<gadget id="calculator">(1/20) * 11</gadget>
<output>11/20 = around 0.55</output>

<gadget id="calculator">1 - (11/20)</gadget>
<output>9/20 = around 0.45</output>

<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(9/20) / (1/30)</gadget>
<output>27/2 = around 13.5</output>

<gadget id="calculator">15 - (27/2)</gadget>
<output>3/2 = around 1.5</output>

<result>3/2 = around 1.5</result>