Brain Teasers wanted

[AbelM]

For a riddle in the magazine of my student association (physics, mathematics and computer science students) i'm looking for some probability riddles. A good example would be this one (http://forumserver.twoplustwo.com/showflat.php?Cat=&Number=1949103&page=0&view=colla psed&sb=5&o=14&fpart=1) which was posted some time ago in this forum.

Another good example would be the famous "three door problem"

Does anyone know some similar riddles? It's quite important that they aren't well known and not easily found on google.

Thanks in advance!

[Gin 'n Tonic]

Not sure how well known this one is, but it's one of my favourites:

Three ladies have their regular lunch at their local restaurant. When they are finished the bill comes to $30 and they each hand the waiter a ten dollar bill.

When the waiter takes the money to the till, the restaurant owner reminds him that there’s a special today with a $5 discount - the bill should only come to $25. The owner puts the $30 in the till and hands the waiter 5 singles to return to the ladies.

On the way to the table, the waiter realizes that he can’t divide up the five singles evenly between the three ladies and also he’s pissed because they never tip – so he slips $2 into his pocket and goes back to the table with the remaining $3.

“Good news ladies” He says. “There’s a 10% discount today – so you all get back a dollar each” and he hands each lady back a dollar.

Now each lady has now paid $9 for the meal, 3x$9 = $27 and there’s $2 in the waiter’s pocket. That makes a total of $29 – what happened to the extra $1 from the original $30 bill?

Enjoy...

[Skjonne]

Lame. You're comparing apples and oranges.

Either: They paid $27 - $25 to the restaurant and $2 to the waiter

Or: They paid $30 - $25 to the restaurant, $2 to the waiter and $3 to themselves

[FatMan]

The error is that the Cents have been taken off the problem. The orignal bill was $25 so each lady owed $8.33 and should have recieved $1.67 in change. The waiter gave them each only a $1 and kept the $0.67 for each lady, so $0.67 * 3 = $2.00. You lost the dollar when you omitted the cents from this problem.

[jason_t]

[ QUOTE ]
Not sure how well known this one is

[/ QUOTE ]

Very.

[jason_t]

This (http://forumserver.twoplustwo.com/showflat.php?Cat=&Board=exchange&Number=2030213&Fo rum=,f20,&Words=-Re%3A&Searchpage=1&Limit=16&Main=2030213&Search=tr ue&where=sub&Name=21723&daterange=1&newerval=18&ne wertype=w&olderval=&oldertype=&bodyprev=#Post20302 13) one is not about probability but no one solved it when I posted it in March.

[-Skeme-]

$30 bill reduced to $25 bill. $5 change. $2 for the water leaves $3 left. $1 to each lady = $30.

[Jazza]

I thought this (http://forumserver.twoplustwo.com/showflat.php?Cat=&Number=2349351&page=1&view=colla psed&sb=5&o=14&fpart=1) was cool, but apparently not many other people did

i'm not sure if this counts as a brainteaser though, it's pretty much a straight up question