Three taxi cabs make a complete trip from downtown to the airport and back in 15, 33 and 55 minutes, respectively. If all three cabs leave at the same time, what is the shortest time that must pass before they are all together again?

165 minutes

Explanation:

We can break the numbers into prime numbers

15 = 3 * 5

33 = 3 * 11

55 = 5 * 11

The shortest time that must pass before they are all together again would be the least multiple that is divisible by all 3 number, which is

3 * 5 * 11 = 165

Hence after 165 minutes the 3 cars should all meet together again.