# Urn problem

1. Dec 12, 2007

### sstudent

1. u have 3 urns and there are black and white marbles in each of them. if u picked a black ball..what is the probabilty it was from the second urn

2. Relevant equations

3. what i did was that i thought the probailty is 1/3 since there is only 3 marbles..but i aint sure if thats right.

2. Dec 12, 2007

### cristo

Staff Emeritus
Could you quote the question exactly as it is given to you, please. For example, in your attempt at a solution you state that there are only 3 marbles, but this is not given in the question.

3. Dec 12, 2007

### HallsofIvy

Staff Emeritus

You haven't given enough information. HOW MANY black and white marbles are there in each urn? (You say there are 3 urns, not three marbles!) If, for example, there exactly the same number of white and black marbles in each urn, knowing the marble was black wouldn't tell you anything about which urn it came from: probability of each would be, as you say, 1/3. But if one urn have more black marbles than white, while the other two had equal numbers, seeing a black marble would tell you that it was more likely that you picked out of that urn.

4. Dec 13, 2007

### Sleek

As everyone said, you have given us insufficient information. But when you get them, try what I say. Its just a hint, but if you get it, then its good, else we're always here to give some more hints or help correct your mistakes.

There are three urns and each having black and white marbles. What do you think is the probability of picking up a black marble from second urn, if you had to? Having known that, can you find the reverse, i.e. if you picked a black marble, what is the probability that it came from second urn?

Regards,
Sleek.

5. Dec 13, 2007

### sstudent

well, it had 10 marbles in the 1st and 2nd urn, and 7 in the 3rd

6. Dec 13, 2007

### Sleek

But have they not given the individual number of black and white marbles in each of them?

Regards,
Sleek.