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Mar 24, 2014
 #32
avatar+2354 
+11
Nobody Special:
reinout-g:

Here are the new probability puzzles!
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Easier one:
A special machine contains 100 red, 100 green and 100 blue marbles.
The machine performs the following four operations.

First, the machine randomly picks 50 marbles from the entire batch.

Secondly the machine divides the marbles into one group of 20 and one group of 30

Furthermore, the machine picks and shuffles 5 marbles of the group of 20 and 10 marbles of the group of 30.

Finally, the machine randomly chooses one of the final 15 marbles it picked and shows it to you.

Assuming you cannot see anything which happens inside the machine.

What are the odds the machine will show you a red marble?

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Alright, I'm gonna \try this one. ^_^
Since there are all 100 marbles per color, and the machine chooses randomly, then there would be an equal chance that you would get any color.
So there would be a 33.33...% chance of red, blue, or green. Right? ^_^

And I'd agree with @stuff on the other one.



You're absolutely right.
Everything the machine does is irrelevant since you cannot see any of the in-between steps.
I'll try and make the easier puzzle a little more difficult when I upload the new ones.
For the answer with the trains, see whether you also understand Bertie's answer.


Reinout
Mar 24, 2014
 #31
avatar+2354 
+11
Bertie:
reinout-g:

Here are the new probability puzzles!
--------------------------------------------------------------------------------------------------------------------------------------
Easier one:
A special machine contains 100 red, 100 green and 100 blue marbles.
The machine performs the following four operations.

First, the machine randomly picks 50 marbles from the entire batch.

Secondly the machine divides the marbles into one group of 20 and one group of 30

Furthermore, the machine picks and shuffles 5 marbles of the group of 20 and 10 marbles of the group of 30.

Finally, the machine randomly chooses one of the final 15 marbles it picked and shows it to you.

Assuming you cannot see anything which happens inside the machine.

What are the odds the machine will show you a red marble?

--------------------------------------------------------------------------------------------------------------------------------------
Hard one :

Steve has two routes (A and B) to travel to his girlfriend which he is indifferent about (see my amazing illustrational skills )

knipsel11.png

Steve has decided to play a game which makes him decide which route to pick.
He always waits where the road splits between route A and B until he either sees a train going from A to B or from B to A.
If the first train Steve sees goes from A to B he always takes route A and if the first train Steve sees is a train going from B to A he always takes route B.
There are equally as many trains going from A to B as there are going from B to A.
Nevertheless Steve has noticed that for every five times he visits his girlfriend he takes route A four times and route B only once.

How can this be?

--------------------------------------------------------------------------------------------------------------------------------------

Reinout-g

edit: I updated a flaw in the illustration, 'work' was meant to be 'girlfriend'



Easier one.
Ignore the flim-flam, via a circuitous route a single marble has been taken at random from the original 300. It's 2 to 1 against it being a red marble.

Harder one.
In principle there can be any number of trains travelling between A and B, (and B and A), but start by thinking in terms of one train in either direction.

Suppose that the train from A to B passes on the hour and the train from B to A at twelve minutes past the hour. If 'our man' arrives at the split in the road at some random time, there will be a 48 minute window within which the next train will be the one from A to B, (12 min past the hour through to 60min past the hour, or 12.01 and 59.99 min past the hour if you're going to be pedantic) and a 12 minute window when the next train will be the one from B to A, (0 min to 12 min). The time interval for A is four times as long as the interval for B so it's four times more likely that the next train will be the A to B train.

Now suppose that there are two trains in either direction, the A to B ones on the hour and half hour, the B to A ones at 6 minutes and 36 minutes past the hour. Combining the windows, there will be a total of 48 minutes, (24+24), during which the next train will be from A to B, and 12 minutes, (6+6), when the next train will be from B to A. Again, that's 4 to 1 in favour of A.

We can do this for higher numbers of trains, though it does become somewhat impracticable after a while !
For three trains in either direction the timings for the A to B trains could be** on the hour, 20 minutes past and 40 minutes past, while the timings for the B to A trains could be at 4 minutes, 24 minutes and 44 minutes past the hour.

** The timings need not be exactly as stated, and the intervals need not be equal. So long as the 'windows' for the A to B trains add to 48 minutes it will always be 4 to 1 in favour of A to B.




Excellent answers Bertie!

Thank you for your descriptive explanation with examples of trains leaving.

In fact, I think I don't even have anything else to add to your answer

I will upload new puzzles shortly
Mar 24, 2014
Mar 23, 2014
 #30
avatar+893 
+11
reinout-g:

Here are the new probability puzzles!
--------------------------------------------------------------------------------------------------------------------------------------
Easier one:
A special machine contains 100 red, 100 green and 100 blue marbles.
The machine performs the following four operations.

First, the machine randomly picks 50 marbles from the entire batch.

Secondly the machine divides the marbles into one group of 20 and one group of 30

Furthermore, the machine picks and shuffles 5 marbles of the group of 20 and 10 marbles of the group of 30.

Finally, the machine randomly chooses one of the final 15 marbles it picked and shows it to you.

Assuming you cannot see anything which happens inside the machine.

What are the odds the machine will show you a red marble?

--------------------------------------------------------------------------------------------------------------------------------------
Hard one :

Steve has two routes (A and B) to travel to his girlfriend which he is indifferent about (see my amazing illustrational skills )

knipsel11.png

Steve has decided to play a game which makes him decide which route to pick.
He always waits where the road splits between route A and B until he either sees a train going from A to B or from B to A.
If the first train Steve sees goes from A to B he always takes route A and if the first train Steve sees is a train going from B to A he always takes route B.
There are equally as many trains going from A to B as there are going from B to A.
Nevertheless Steve has noticed that for every five times he visits his girlfriend he takes route A four times and route B only once.

How can this be?

--------------------------------------------------------------------------------------------------------------------------------------

Reinout-g

edit: I updated a flaw in the illustration, 'work' was meant to be 'girlfriend'



Easier one.
Ignore the flim-flam, via a circuitous route a single marble has been taken at random from the original 300. It's 2 to 1 against it being a red marble.

Harder one.
In principle there can be any number of trains travelling between A and B, (and B and A), but start by thinking in terms of one train in either direction.

Suppose that the train from A to B passes on the hour and the train from B to A at twelve minutes past the hour. If 'our man' arrives at the split in the road at some random time, there will be a 48 minute window within which the next train will be the one from A to B, (12 min past the hour through to 60min past the hour, or 12.01 and 59.99 min past the hour if you're going to be pedantic) and a 12 minute window when the next train will be the one from B to A, (0 min to 12 min). The time interval for A is four times as long as the interval for B so it's four times more likely that the next train will be the A to B train.

Now suppose that there are two trains in either direction, the A to B ones on the hour and half hour, the B to A ones at 6 minutes and 36 minutes past the hour. Combining the windows, there will be a total of 48 minutes, (24+24), during which the next train will be from A to B, and 12 minutes, (6+6), when the next train will be from B to A. Again, that's 4 to 1 in favour of A.

We can do this for higher numbers of trains, though it does become somewhat impracticable after a while !
For three trains in either direction the timings for the A to B trains could be** on the hour, 20 minutes past and 40 minutes past, while the timings for the B to A trains could be at 4 minutes, 24 minutes and 44 minutes past the hour.

** The timings need not be exactly as stated, and the intervals need not be equal. So long as the 'windows' for the A to B trains add to 48 minutes it will always be 4 to 1 in favour of A to B.
Mar 23, 2014
 #79
avatar+118728 
+9
Hi all,
There weren't many questions today, it was Saturday/Sunday after all but there were still lots of great answers provided by CPhill, Radamus, Kitty<3, Alan, Millie, Rom, Reinout-g, Dms and Peter. Thanks folks!

Every day there seems to be one or two more people offering solutions. I think that this is great! It has always been my philosophy that everyone should be encouraged to join in. If you are not sure if your answer is correct you should say so BUT I think that the person who asks the question needs to accept some responsibiliy for deciding whether to accept or reject another person's offered solution. Also, almost all answers, especially lower level ones are checked by other more senior mathematicians so very few wrong answers will escape detection. If you are not happy with the answer you are given you can always 'bump' your own question and ask for someone else to take a look at it, but it may be to your advantage to wait a little while first.

Nobody Special has attempted one of reinout's puzzles. I am really pleased, I wish more people would show interest in the puzzles. Reinout is not always around so you may have to wait a little while for him to comment on your answer.

Today Reinout 'proved' that 1=2. He was illustrating what can happen when you accidentally divide by 0. I thought it was great!
This was the thread. http://web2.0calc.com/questions/i-need-help-with-this-ecvation

Information for new people - Private Messages.
There is a private message system here. If you left click on your username you will find it. If you go into the user control panel then Board Preferences, you can get a pop-up display for new messages, that is if you wish to do so.
If you send a message it will sit in your outbox until the recipient opens it. In the mean time you can edit it or delete it if you wish. It only moves to sent mail after the recipient opens it. If you delete the message the recipient will still get a message but it will just say 'message deleted before delivery' It is a really cool system. I wish my usual email worked this way.

That is it for today.
Enjoy what is left of your weekend.
Melody.
Mar 23, 2014
 #3
avatar+118728 
0
Mar 23, 2014
 #2
avatar+118728 
+1
Mar 23, 2014
 #2
avatar+118728 
0
Mar 23, 2014

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