The distance between A and B is 600 kilometers. The bus and the truck depart from the two cities at the same time. The bus travels 60 kilometers per hour, which is 15 kilometers faster than the truck. After 4 hours, the truck arrives at point C between A and B. The bus How many hours will it take to get to point C?
甲、乙两城相距600千米，客车和货车同时从两城相对开出，客车每小时60千米，比货车快15千米，经过4小时货车到达甲乙两地之间的丙地，客车还要行多少小时才能到达丙地．
<gadget id="calculator">60 - 15</gadget>
<output>45</output>

<gadget id="calculator">45 * 4</gadget>
<output>180</output>

<gadget id="calculator">600 - 180</gadget>
<output>420</output>

<gadget id="calculator">420 / 60</gadget>
<output>7</output>

<gadget id="calculator">7 - 4</gadget>
<output>3</output>

<result>3</result>