## Welcome to BrainDen.com - Brain Teasers Forum

 Welcome to BrainDen.com - Brain Teasers Forum. Like most online communities you must register to post in our community, but don't worry this is a simple free process. To be a part of BrainDen Forums you may create a new account or sign in if you already have an account. As a member you could start new topics, reply to others, subscribe to topics/forums to get automatic updates, get your own profile and make new friends. Of course, you can also enjoy our collection of amazing optical illusions and cool math games. If you like our site, you may support us by simply clicking Google "+1" or Facebook "Like" buttons at the top. If you have a website, we would appreciate a little link to BrainDen. Thanks and enjoy the Den :-)
Guest Message by DevFuse

57 replies to this topic

### #1 octopuppy

octopuppy

Senior Member

• VIP
• 1303 posts
• Gender:Male

Posted 18 March 2008 - 07:18 PM

Every now and then you see a puzzle, peek at the answer, then wish you'd taken the time to solve it instead of peeking. I did that with this puzzle, but I won't post the solution now so you don't make the same mistake. Enjoy!

A river has six islands in the middle, connected by 13 bridges as shown:

Enemy tanks are approaching on the other side of the river, so you've set up explosives to blow up the bridges. Unfortunately your detonators came from a dodgy batch, so each bridge has only a 50% chance of blowing up (independently of the others). What is the probability that the enemy tanks will still be able to find a route across the river?
Spoiler for Hint

EDIT: When you've solved this, there's a related, more general question to consider here...
• 0

### #2 Noct

Noct

• Members
• 388 posts

Posted 18 March 2008 - 07:35 PM

Spoiler for Not sure if it's right, but...

• 0

### #3 octopuppy

octopuppy

Senior Member

• VIP
• 1303 posts
• Gender:Male

Posted 18 March 2008 - 09:30 PM

Spoiler for Not sure if it's right, but...

No guessing! It's how you work it out that matters...
• 0

### #4 Seventh Sage

Seventh Sage

• Members
• 131 posts

Posted 18 March 2008 - 09:45 PM

Every now and then you see a puzzle, peek at the answer, then wish you'd taken the time to solve it instead of peeking. I did that with this puzzle, but I won't post the solution now so you don't make the same mistake. Enjoy!

A river has six islands in the middle, connected by 13 bridges as shown:

Enemy tanks are approaching on the other side of the river, so you've set up explosives to blow up the bridges. Unfortunately your detonators came from a dodgy batch, so each bridge has only a 50% chance of blowing up (independently of the others). What is the probability that the enemy tanks will still be able to find a route across the river?

Spoiler for Hint

Spoiler for If you think about it...

• 0

### #5 Noct

Noct

• Members
• 388 posts

Posted 18 March 2008 - 10:45 PM

No guessing! It's how you work it out that matters...

Who said anything about a guess?
• 0

### #6 Noct

Noct

• Members
• 388 posts

Posted 18 March 2008 - 10:49 PM

Spoiler for If you think about it...

I think you got something wrong. It sounds like you're saying there's a chance that a die might land with 5 up, and an equal chance that it won't land with any other number up, so they must equal 100% so it must be 50% that the die will land with a 5 up.

EDIT--
Wait, i think i just understand what you were trying to say. You're wording is a little confusing, but i do see what you're saying.

But i still think your reasoning is faulty

Edited by Noct, 18 March 2008 - 10:52 PM.

• 0

### #7 trojan2k8

trojan2k8

Junior Member

• Members
• 26 posts

Posted 18 March 2008 - 10:52 PM

i would say

• 0

### #8 Noct

Noct

• Members
• 388 posts

Posted 18 March 2008 - 10:54 PM

i would say

But that would be the chance of just three bridges across. We also have to add in the probabilities of 4+ bridges (i believe)
• 0

### #9 strykr1a0

strykr1a0

Junior Member

• Members
• 21 posts

Posted 18 March 2008 - 11:02 PM

Spoiler for If you think about it...

Bravo, I am very very impressed
• 0

### #10 statman

statman

• Members
• 124 posts

Posted 18 March 2008 - 11:49 PM

Every now and then you see a puzzle, peek at the answer, then wish you'd taken the time to solve it instead of peeking. I did that with this puzzle, but I won't post the solution now so you don't make the same mistake. Enjoy!

A river has six islands in the middle, connected by 13 bridges as shown:
Enemy tanks are approaching on the other side of the river, so you've set up explosives to blow up the bridges. Unfortunately your detonators came from a dodgy batch, so each bridge has only a 50% chance of blowing up (independently of the others). What is the probability that the enemy tanks will still be able to find a route across the river?

Spoiler for Hint

Spoiler for Guessing not allowed?

Edited by statman, 18 March 2008 - 11:51 PM.

• 0

#### 0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users