Math: It takes 20 seconds for a train to enter a 300-meter-long tunnel and leave completely. The time for the train to stay in the tunnel completely is 10 second. How long is the train?
Set the time to x? The train should be x meters long.
Solution: Let the train be x meters long.
(300+x)/20=(300-x)/ 10
300+x=(300-x)/ 10*20
300+x=(300-x)*2
300+x=600-2x
3x=300
x= 100
A: ...