"A" and "B" run around a circular track

Hi All,

This prompt gives us a number of facts to work with:
1) "A" and "B" run around a circular track starting from the same point simultaneously (and in the same direction)
2) "A" meets "B" for the first time when "A" is exactly in the middle of his 5th round (meaning 4.5 laps complete)
3) "A" is faster than "B" and take 70 seconds to complete 1 lap.

We're asked for the amount of time it takes for B take to complete 1 lap.

Since A is faster than B, we know that they will meet when A has 'lapped' B. Thus, when A has completed 4.5 laps, B has completed just 3.5 laps. We can determine A's total time at 4.5 laps:

(4.5 laps)(70 secs/lap) = 315 seconds

This is the same amount of time that B has been running, so we can determine B's speed:

(3.5 laps)(X secs/lap) = 315 seconds
X = 315/3.5
X = 630/7
X = 90 seconds/lap

Final Answer: A

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