Two trains are travelling on parallel tracks, one towards the other. The speed of each is $20m/s$. Train number one is $100m$ long and train number $2$ is $200m$ long.
They meet and pass each other. How long does train $2$ travel past the front of train $1$?
I got this:
Because the trains travel at $20m/s$, the result is the same as if one train was going $40m/s$ and the other one wasn't moving.
The train moves for $200m$ to completely pass the front of the other one.
$$\text{speed}=\frac{\text{length}}{\text{time}}$$
$$\text{time}=\frac{\text{length}}{\text{speed}}$$
$$\text{time}=\frac{200m}{40m/s}$$
$$\text{time}=5s$$
This is a problem from a workbook,so I checked the solution.It says $7,5$s.
I think this is actually the time the trains need to pass each other completely. $$40m/s*7,5s=300m$$ Here $300m$ is the total of both lengths.
Please tell me if I am wrong and if yes what did I miss.