The highway from Tieling to Harbin is 450 kilometers long, and a car travels 180 kilometers in 2 hours. At this speed, how many hours will it take to reach Harbin? (First use arithmetic method to solve, and then use proportional knowledge to solve)
铁岭到哈尔滨的公路长是450千米，一辆汽车2小时行180千米．照这样的速度，还要几小时才能到达哈尔滨？(先用算术方法解，再用比例知识解)
<gadget id="calculator">450 - 180</gadget>
<output>270</output>

<gadget id="calculator">180 / 2</gadget>
<output>90</output>

<gadget id="calculator">270 / 90</gadget>
<output>3</output>

<result>3</result>