The road from city A to city B is 315 kilometers long. The speed of a car on the expressway is 90 kilometers per hour, and the speed on the ordinary road is 60 kilometers per hour. How much time does this car save by taking the expressway from city A to city B instead of taking ordinary roads?
甲城到乙城的公路长315千米，一辆车走高速公路的速度是每小时90千米，走普通公路的速度是每小时60千米．这辆汽车从甲城到乙城走高速公路比走普通公路节省多少时间？
<gadget id="calculator">315 / 60</gadget>
<output>21/4 = around 5.25</output>

<gadget id="calculator">315 / 90</gadget>
<output>7/2 = around 3.5</output>

<gadget id="calculator">(21/4) - (7/2)</gadget>
<output>7/4 = around 1.75</output>

<result>7/4 = around 1.75</result>