The effect of both trains moving in opposite directions is the same that if one of them was stationary and the other one was moving at 65 + 72 = 137 mph.
Now, we know that distance = speed x time. Let's call the distance d and the time y
We would have:
[tex]d=137t[/tex]Using the distance we're given,
[tex]822=137t[/tex]Solving for t :
[tex]\begin{gathered} 822=137t \\ \rightarrow\frac{822}{137}=t \\ \Rightarrow t=6 \end{gathered}[/tex]It would take 6 hours for them to be 822 miles apart .
