Both cars depart from a certain city to Shehong Foreign Language School. The shuttle bus travels 50 kilometers per hour and arrives in 4 hours, and the private car travels 80 kilometers per hour. If the shuttle bus and the private car arrive at the school at the same time, how many hours in advance should the shuttle bus leave?
两辆汽车都从某一城市出发到射洪外国语学校．班线车每小时行50千米，4小时到达，私家车每小时行80千米，如果班线车和私家车要同时到达学校，班线车要提前几小时出发？
<gadget id="calculator">4 / 80</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">50 * (1/20)</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">4 - (5/2)</gadget>
<output>3/2 = around 1.5</output>

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