Andy, Bob, Carl, and David are trying to cross a river to escape an island with an evil dictator. However, they only have one canoe that can fit 2 people at a time. From practice, they know their time it takes to cross the river. Andy can cross in 1 minute, Bob in 2 minutes, Carl in 5 minutes, and David the slowpoke can cross in 10 minutes. 

The dictator decided to give them a chance, but states a rule: when there is 2 people on the canoe, only the person with the slower cross time can row the boat. If all four can get across in 17 minutes or less, they will be granted freedom, otherwise they will be killed. They cannot swim across or use any tricks or other methods. 

Can figure out a way to get them all across in 17 minutes?

– Andy and Bob go across (takes 2 minutes, because Bob’s row time is 2 minutes)

– Andy rows back (takes 1 minute, total time 3 minutes)

– Andy and Carl go across (takes 5 minutes, total time 8 minutes)

– Andy rows back (takes 1 minute, total time 9 minutes)

– Andy and David go across (takes 10 minutes, total time 19 minutes)

However that is too long! They will be killed by the evil dictator. They must find a way to get across in 17 minutes or less.

The trick is to realize that the Carl (5 min) and David (10 min) must go together to reduce time. Here is the method to get across:

 

– Andy and Bob go across (takes 2 minutes)

– Andy rows back (takes 1 minute, total time 3 minutes)

– Carl and David go across (takes 10 minutes, total time 13 minutes)

– Bob rows back (takes 2 minutes, total time 15 minutes)

– Andy and Bob go across (takes 2 minutes, total time 17 minutes)

They all got across safely!

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