solution: b.

Now I will express the positions of the two trains as functions of time so that I can solve for when they are equal to each other. tex2html_wrap_inline46
Train A: tex2html_wrap_inline48 , tex2html_wrap_inline50 , x=30t-4.0
Train B: tex2html_wrap_inline54 , tex2html_wrap_inline56 , x=-35t+6.0
When they cross paths, their positions (x) will be equal.
Set the two equations equal to each other and solve for t.

eqnarray32

tex2html_wrap62