Math Is Fun Forum

charlson

Guest

bags of marbles rethinking

the answer should hve been 1/2, bcoz its against bag A and bag C. they both have a white marble on them,which you already have in your hand. pulling out the 2nd marble will determine if its bag A or bag B... does anyone agree?

Bob

Administrator

Registered: 2010-06-20

Posts: 10,643

Re: bags of marbles rethinking

hi charlson

Welcome to the forum.

This is known as a conditional probability problem.  The correct result can catch you out.

Let's say the three white marbles can be told apart in some way.  eg.  We write 1, 2 and 3 on them.  I'll refer to them as W1, W2, and W3.  It doesn't matter about the blacks.

Since we know we have a white it could be W1, or W2 or W3.

If it is W1, then the other marble is also white (W2).

If it is W2, then the other marble is also white (W1).

Only if it is W3 is the other marble B.

So there are 2 chances out of 3 of getting a white, not 1 chance in 2. Sorry.

Bob

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Children are not defined by school ...........The Fonz
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Sometimes I deliberately make mistakes, just to test you!  …………….Bob