Imagine that you have a bag with a number of white balls and a number of black balls, such that when picking two random balls from the bag, the probability of them being the same color is the same as the probability of them being of different colors. How many balls of each color are there? (Edit: the correct answer is the one using the fewest number of balls)

It's easy to find the answer by trying the possibilities. If you want something a bit harder, make a formula for calculating the likelihood of picking two balls of the same color given n black balls and m white balls.

## Easy: Balls and probabillity

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### Easy: Balls and probabillity

Last edited by userxp on Thu Sep 17, 2009 6:52 pm UTC, edited 1 time in total.

- skeptical scientist
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### Re: Easy: Balls and probabillity

**Spoiler:**

I'm looking forward to the day when the SNES emulator on my computer works by emulating the elementary particles in an actual, physical box with Nintendo stamped on the side.

"With math, all things are possible." —Rebecca Watson

"With math, all things are possible." —Rebecca Watson

### Re: Easy: Balls and probabillity

This is my working, differs a bit from skep's:

**Spoiler:**

doogly wrote:Oh yea, obviously they wouldn't know Griffiths from Sakurai if I were throwing them at them.

### Re: Easy: Balls and probabillity

dedalus wrote:This is my working, differs a bit from skep's:Spoiler:

The balls are taken out without replacement. Therefore:

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### Re: Easy: Balls and probabillity

**Spoiler:**

- skeptical scientist
- closed-minded spiritualist
**Posts:**6142**Joined:**Tue Nov 28, 2006 6:09 am UTC**Location:**San Francisco

### Re: Easy: Balls and probabillity

**Spoiler:**

I'm looking forward to the day when the SNES emulator on my computer works by emulating the elementary particles in an actual, physical box with Nintendo stamped on the side.

"With math, all things are possible." —Rebecca Watson

"With math, all things are possible." —Rebecca Watson

### Re: Easy: Balls and probabillity

jaap wrote:dedalus wrote:This is my working, differs a bit from skep's:Spoiler:

The balls are taken out without replacement. Therefore:Spoiler:

Ah of course. Thanks for the correction.

doogly wrote:Oh yea, obviously they wouldn't know Griffiths from Sakurai if I were throwing them at them.

### Re: Easy: Balls and probabillity

**Spoiler:**

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