# Can you solve the river crossing riddle? - Lisa Winer | Summary and Q&A

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November 1, 2016
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TED-Ed
Can you solve the river crossing riddle? - Lisa Winer

## TL;DR

Lions and wildebeest must cross a crocodile-infested river to escape a wildfire, without letting the lions outnumber the wildebeest on either side. The fastest solution requires eleven crossings.

## Key Insights

• đĻ Strategic decision-making is crucial to ensure the survival of both lions and wildebeest.
• đĻ The solution involves alternating the crossings between lions and wildebeest to maintain balance and prevent the lions from overpowering.
• #ī¸âŖ Safety in numbers is a key factor in the success of the animals' escape plan.
• đ The raft serves as a crucial tool for transportation, but its usage must be carefully managed.
• â The animals' survival instincts significantly impact their behavior and the course of action.
• â Lion-wildebeest pairs are strategically sent back to avoid outnumbering on either side.
• đŠī¸ The smallest number of crossings required to ensure all animals' safety is eleven.

## Transcript

As a wildfire rages through the grasslands, three lions and three wildebeest flee for their lives. To escape the inferno, they must cross over to the left bank of a crocodile-infested river. Fortunately, there happens to be a raft nearby. It can carry up to two animals at a time, and needs as least one lion or wildebeest on board to row it across t... Read More

### Q: What challenge do the lions and wildebeest face?

The animals must cross a river infested with crocodiles to escape a wildfire.

### Q: How many animals can the raft carry at a time?

The raft can carry up to two animals at a time.

### Q: What happens if the lions ever outnumber the wildebeest?

If the lions outnumber the wildebeest on either side, their instincts will kick in, and the lions will attack the wildebeest.

### Q: How many crossings are needed to get all the animals across safely?

The fastest solution requires eleven crossings to ensure all animals reach the left bank without the lions overpowering the wildebeest.

## Summary

In this video, a compelling problem is presented: three lions and three wildebeest need to cross a crocodile-infested river to escape a raging wildfire. However, the lions must never outnumber the wildebeest on either side. The task is to determine the fastest way for all six animals to cross without the lions causing harm.

### Q: What is the problem presented in the video?

The problem is that three lions and three wildebeest need to cross a river without the lions ever outnumbering the wildebeest on either side.

### Q: What is the consequence if the lions outnumber the wildebeest on either side?

If the lions outnumber the wildebeest, their instincts will kick in, and the lions will cause harm.

### Q: How many animals can the raft carry at a time?

The raft can carry up to two animals at a time.

### Q: What options are there for the first crossing?

The options for the first crossing are one wildebeest, one lion, two wildebeest, two lions, or one of each.

### Q: Why are the options of one animal going alone or two wildebeest crossing first not viable?

If one animal goes alone, they would have to come straight back, and if two wildebeest cross first, the remaining one would immediately get eaten.

### Q: What happens if one of each animal crosses first?

If one of each animal crosses first, the wildebeest on the raft will be outnumbered as soon as it reaches the other side.

### Q: What is the recommended second option for the first crossing?

The recommended second option is to have one of each animal cross first.

### Q: What happens if the wildebeest stays and the lion returns after the first crossing?

If the wildebeest stays and the lion returns, there will be three lions on the right bank, which is bad news for the two remaining wildebeest.

### Q: How many options are there for the third crossing?

There are five options for the third crossing, but with one lion already on the left bank.

### Q: What happens if two wildebeest go or one of each animal goes for the third crossing?

If two wildebeest go, the one that stays will get eaten. If one of each animal goes, the wildebeest on the raft will be outnumbered as soon as it reaches the other side.

### Q: What is the solution for the third crossing?

The solution for the third crossing is to send only the two lions.

### Q: What happens after the third crossing?

After the third crossing, two lions are left on the left bank, and the wildebeest are waiting on the right bank.

### Q: What is the next step after the third crossing?

The next step is for two wildebeest to cross.

### Q: What is the next step after two wildebeest cross?

After two wildebeest cross, there is no sense in sending them back or two lions back.

### Q: What is the next trip after the two wildebeest cross?

The next trip should either be a pair of lions or a pair of wildebeest.

### Q: Why can't the lions go back after the wildebeest cross?

If the lions go back, they would outnumber the wildebeest on the right bank.

### Q: What happens after the wildebeest cross?

After the wildebeest cross, there is just one lion left on the left bank.

### Q: How do the remaining animals cross the river?

The remaining animals cross the river by having the one lion raft back and bring the fellow lions over one by one.

### Q: How many trips does it take to get everyone across safely?

It takes a total of eleven trips to get everyone across safely.

## Takeaways

In solving this problem, it is essential to list all the decisions that can be made at each point and consider the consequences of each choice. The optimal solution involves sending one of each animal first, then having a lion stay on the left bank while the wildebeest goes back to the right. Subsequently, the next crossings involve sending two lions, two wildebeest, and finally one lion rafting back and bringing the remaining lions over one by one. The solution requires eleven trips in total, ensuring the safety of all the animals.

## Summary & Key Takeaways

• In order to escape a wildfire, three lions and three wildebeest must cross a crocodile-infested river to the left bank.

• A nearby raft can carry two animals at a time, but at least one lion or wildebeest must be on board to row it.

• The fastest solution involves strategic crossings, ensuring the lions never outnumber the wildebeest on either side.