Cars A and B start from both places A and B at the same time, and they will meet at a certain moment. Because car A departs a certain period of time earlier, the two cars meet 30 minutes earlier. It is known that car A's speed is 60 kilometers per hour, and car B's speed is 40 kilometers per hour, how many minutes earlier did car A start?
甲、乙两车从A、B两地同时出发，某时刻将相遇由于甲车提前一段时间出发，结果两车相遇提前了30分钟，已知甲车速度每小时60千米，乙车速度每小时40千米，那么甲车提前了多少分钟出发．
<result>50</result>