Two race cars are speeding around the racetrack at different speeds. One goes around the track in 2 minutes; the other goes around the track in 11/2 minutes. How often does the faster car pass the slower car?

one car takes 2 minutes for once around
so in 1 minute it does a 1/2 around
 .
second car takes 1 1/2 minutes for once around ( 1 1/2 = 3/2)
so in 1 minute it does 2/3 round 
 every minute the second car gains 2/3 -1/2 of the round
2/3 = 4/6, 1/2 = 3/64/6 - 3/6 = 1/6 of the track per minute
so every 6 minutes the faster car overtakes the slower car.