B It takes 6 hours to work alone, and the efficiency is 1/6 hours.

It takes X hours for Party A and Party B to cooperate.

X = (1-1/8)/(1/4+1/6) = 7/4 hours.

2. let the speed of the train be x.

Because both Party A and Party B are moving when the train passes by, we should use the relative speed, because the unit used by the car when passing by Party A and Party B is seconds, so the speed of two people should also be converted into seconds.

3.6 kilometers per hour = per second 10 meter.

Take A as stationary, and the speed of the train passing through A is x+ 10.

Think of B as stationary, and the speed of the train passing through B is x- 10.

Make an equation according to the length of the train.

(x- 10)* 17 =(x+ 10)* 15

X =160m per second

So the length of the train is 2550 meters.