.
From the condition, the train' speed (rate) is meters per second.
To go completely through the tunnel, the train must move forward for the length 400+x meters.
It takes exactly 30 seconds, so the distance it moves forward for this time is
Rate*Time = = 3x.
So, your equation is
400+x = 3x, (saying that the two distances are the same)
400 = 2x ====> x = = 200 meters.
Answer. The train is 200 meters long.
There is a bunch of lessons "on the train", as a part of Travel and Distance problems.
See the lessons
- A train passing a telegraph post and passing a bridge
- A train passing a platform
- A train passing through a tunnel
- A light-rail train passing a walking person
- A train passing another train
in this site.
Also, you have this free of charge online textbook in ALGEBRA-I in this site
ALGEBRA-I - YOUR ONLINE TEXTBOOK.
The referred lessons are the part of this textbook under the section "Word problems", the topic "Travel and Distance problems".
Save the link to this online textbook together with its description
Free of charge online textbook in ALGEBRA-I
https://www.algebra.com/algebra/homework/quadratic/lessons/ALGEBRA-I-YOUR-ONLINE-TEXTBOOK.lesson
to your archive and use it when it is needed.