Question 402625
Let x = rate of one car
y = rate of another car.

Then 18x + 18y = 1, and 
90x - 90y = 1

Multiply the top equation by 5 and add to the bottom equation:

180x = 6, or x = 1/30 mile per second, or 2 miles per hour.

==> y = 1/45 mile per second, or {{{1&1/3}}} miles per hour.