Next to the railway, there is a highway along the direction of the railway. On the highway, a car is running at the speed of 10 meters per second. At this time, a train is running at the speed of 21 meters per second It took 2.5 seconds to drive from the back, from the front to the back, past the car. How long is this train?

Next to the railway, there is a highway along the direction of the railway. On the highway, a car is running at the speed of 10 meters per second. At this time, a train is running at the speed of 21 meters per second It took 2.5 seconds to drive from the back, from the front to the back, past the car. How long is this train?

（21-10）×2.5
=11×2.5
=27.5m

There are eight stations on a certain railway line, and there are several different tickets on this railway line

(7 + 6 + 5 + 4 + 3 + 2 + 1) * 2 = 56 species, right?

A passenger railway line, a total of 7 stations, asked to prepare () kinds of tickets
emergency

It's a matter of statistics
That is to say, two of the seven stations are selected to form a ticket
The formula thus listed is C (7) (2) = 7 * 6 / 2 = 21
Where: C (7) (2) means that 7 is in the lower right corner of C and 2 is in the upper right corner of C
So if it's one-way, you need to prepare 21 kinds of tickets
If it's a round trip, you need 21 * 2 = 42 kinds of tickets
According to the meaning of the title should be round-trip, so you need to prepare 42 kinds of tickets
Good luck!