Hi guerrero,

This is not something that you'd be likely to see on the GMAT. There are a couple of ways to solve it though - you COULD map out the various times and see if the two buses ever hit the same time. Here's a more elegant way though:

Since there are 24 hours in a day, we have to account for THAT repeating cycle.

3pm = 15th hour +10 hour "blocks" (we'll call these x)
4pm = 16th hour +15 hour "blocks" (we'll call these y)

It's worth noting that x and y ARE INTEGERS.

So, we have to figure out IF (notice the word IF)

15 +10x = 16 + 15y

10x - 15y = 1

x and y have to be integers, so there's NO WAY for 10x - 15y to = 1

Final Answer: E

GMAT assassins aren't born, they're made,
Rich