Xiaoming and Xiaoli set out from a and B, 8km apart, and walked in opposite directions. It is known that Xiaoming's speed is 50M / min, and Xiaoli's speed is 40m / min. one hour after Xiaoming set out, Xiaoli started to set out with a pet dog. The dog shuttled back and forth between them until they met. How many kilometers did the dog run? (the known speed of the dog is 90m / min)

Xiaoming and Xiaoli set out from a and B, 8km apart, and walked in opposite directions. It is known that Xiaoming's speed is 50M / min, and Xiaoli's speed is 40m / min. one hour after Xiaoming set out, Xiaoli started to set out with a pet dog. The dog shuttled back and forth between them until they met. How many kilometers did the dog run? (the known speed of the dog is 90m / min)

The following is my analysis process:
8 km = 8000 m
The dog didn't stop running until they met
(so) let Xiao Li and Xiao Ming meet in X minutes
(because Xiao Ming starts one hour first, the distance is 50 × 60 meters.)
50×60+50x+40x=8000
3000+90x=8000
90x=5000
X = 500 / 9 (500 out of 9)
The meeting time is 500 / 9 minutes
The dog ran 500 / 9 minutes
The dog ran 500 / 9 × 90 = 5000 meters = 5 kilometers
A: the dog ran a total of 5 kilometers
Believe me. The teacher just changed it. It's right!