The distance between A and B is 120 km. A bus departs from A to B. Start at 50 kilometers per hour, then slow down to 40 kilometers per hour. One hour after the bus departs, a car also departs from A to B and travels 80 kilometers per hour. As a result, the two cars arrive at B at the same time. How long does it take for the bus to start from A before slowing down?
甲、乙两地相距120千米．一辆大客车从甲地出发前往乙地．开始时每小时行50千米，中途减速为每小时行40千米．大客车出发l小时后，一辆小轿车也从甲地出发前往乙地，每小时行80千米，结果两辆车同时到达乙地，问大客车从甲地出发多少时间后才降低速度？
<gadget id="calculator">120 / 80</gadget>
<output>3/2 = around 1.5</output>

<gadget id="calculator">1 + (3/2)</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">40 * (5/2)</gadget>
<output>100</output>

<gadget id="calculator">120 - 100</gadget>
<output>20</output>

<gadget id="calculator">50 - 40</gadget>
<output>10</output>

<gadget id="calculator">20 / 10</gadget>
<output>2</output>

<result>2</result>