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## Comment on

Total Number of Balls## to save some time and exempt

## Great approach!

Great approach!

## It seems like using

## Solving quadratic equations

Solving quadratic equations is an important skill to have on test day.

## hi Brent

When I'm solving a quadratic equation, why can't the answer be N=8 and N=-9 instead?

## You're right to say the

If you derived the equation N² - N - 72 = 0 , then the solutions cannot be 8 and -9.

We can verify this by plugging N = 8 and N = -9 into the equation.

For example, if N = 8, we get: 8² - 8 - 72 = 0

Simplify to get: -16 = 0. Doesn't work.

To solve the equation N² - N - 72 = 0 we must first...

Factor the left side to get: (N - 9)(N + 8) = 0

So, either N = 9 or N = -8

Does that help?

Cheers,

Brent

## Hi Brent,

What if there is a question where 2 balls are drawn at the same time. How should we approach such questions?

## The answer (and the solution)

The answer (and the solution) would be exactly the same. Here's why:

Let's say Person A reaches into the box and grabs 2 balls (1 ball with each hand) and removes them at the EXACT SAME TIME.

For this scenario, P(both balls are white) = some number

Now replace the two balls and restart the experiment.

This time, Person B reaches into the box and grabs 2 balls (1 ball with each hand) and TRIES remove them at the same time, BUT his left hand (holding a ball) leaves the box 0.0000000001 seconds before the right hand leaves the box.

Should it make a difference that the left hand exited the box 0.0000000001 seconds before the right hand?

No. It makes no difference.

So, for any question in which two or more items are selected (without replacement), it makes no difference to our final answer if we assume one object was removed before the other object(s).

Does that help?

## Yes, thanks!

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