Explanation

The question asks (indirectly) how far the two cars traveled, as those distances are necessary to find the distance between them. Since the cars go in the same direction, the skeleton equation is as follows:

Car A’s distance – Car B’s distance = Distance between cars

All distances refer to the time when car A ran out of fuel.

Since the limiting factor in this case is A’s fuel supply, calculate how far the car is able to drive before running out of fuel. This in itself is a rate problem of sorts:

30 miles per gallon × 8 gallons = 240 miles

So car A will end up 240 miles north of its starting point, which happens 240 ÷ 40 = 6 hours after it started. What about car B? It started an hour later and thus traveled (30 miles per hour)(6 hours – 1 hour) = 150 miles by that time.

Therefore, the two cars were 240 – 150 = 90 miles apart when car A ran out of fuel.