First of all, we need to know what the encounter problem is. ? The problem of meeting is that two moving objects start from two places at the same time, move in opposite directions and meet on the way. This kind of application problems are all encountered problems.

Business with problems:

Meeting time = total distance ÷ (speed A+ speed b)?

Total distance = (speed A+ speed B) × meeting time?

Let's solve this problem: there are two situations.

First: B caught up with C, and they met. At this time, the distance between a and BC is equal.

The time for B to catch up with C is 280÷(80-72)=35 minutes.

The second case: A catches up with B first, and then between B and C. ..

Suppose D is driving at the average speed of B and C, and the speed is: (80+72)÷2=76, and D is in the middle distance between B and C at the beginning, then when BCD starts driving, D is always between B and C, because the speed of D is their average.

Then the title is understood as: the time for A to catch up with D, and the distance for AD is: 280+ 140=420.

So the catch-up time is: 420÷(90-76)=30.

So the final answer is: 30 minutes, the second case.