9. Solution: Let the wall area to be painted in each room be x square meters. According to the meaning of the question, you must

8x-50/3- 10x+40/5 = 10

10x=520

x=52

1 1. Solution: (1) The distance from the front of the train to the rear of the train is x meters, and the average speed of the train during this time is x/ 10.

(2) The distance from the train entering the front of the tunnel to the train leaving the back of the tunnel is (300+x) meters, and the average speed of the train during this time is 300+x/20.

(3) No change

(4) According to the meaning of the question, it is 300+x/20=x/ 10.

x=300

The length of this train is 300 meters.