I know we dont need to solve the problem all the way through, but I like to do so in practice, just in case I get a similar PS problem. I try to stick with the same model when it comes to rate problems, using R*T=D. Can you set up an algebraic equation for this problem that would give a solution?
Say, after h hours, the distance between the two cars is 2 miles.
Thus, 50h - 40h + 1 (initially, the two cars were 1 mile apart) = 2
or, 10h = 1
or, h = 1/10.....sufficient.
in 3 minutes, car X travelled 3X miles and car Y travelled Y miles. Also, the two cars were 0.5 miles away.
Hence, 3X - 3Y + 0.5 = 1
or, 3X - 3Y = 0.5 -----(1)
Say, the two cars will be 2 miles apart in h hours from now.
Thus, Xh - Yh + 1 = 2
or, h(X-Y) = 1 ------(2)
Using equations (1) and (2), One can get h.....hence, sufficient.
Hence, D should be the answer.
I hope, this is what you were looking for.