Another interesting problem: crossing the bridge

Every once in a while I like to propose some interesting problems, and this one certainly fits the profile:

• Four friends need to cross a bridge
• A maximum of two people can cross the bridge at the same time
• It's night and it's very dark, so they use the only lamp they have, to see the way
• Each pair can only cross the bridge at the rate of the slower person (each person's crossing rate is given by the number showed on top of each person)
• Try to make them all cross the bridge in a total of 17 minutes

Yes, 17 minutes. It is possible! Here's a snapshot of the result...just to tease you :-)

If you got 21...that's not bad! That's the second fastest solution. But 17 is still possible and it's the fastest solution. If you really want to cheat know the solution check out this picture that's hiding the solution.