Cars A and B start from places A and B at the same time and drive towards each other. It is known that the speed of car A is 60km/hour and the speed of car B is 40km/h. Depart at some time, how much time should car A depart in advance?
甲，乙两车从A，B两地同时出发，相向而行，已知甲车速度是60km/时，乙车速度是40km/h，如果要使两车提前30分钟相遇，让甲车提前一些时间出发，甲车应提前多少时间出发？
<gadget id="calculator">60 + 40</gadget>
<output>100</output>

<gadget id="calculator">100 * 0.5</gadget>
<output>50</output>

<result>50</result>