9 coins: 7H and 2T

**dwsmith** · April 17th 2010, 05:27 PM

p[of tossing nine coins and getting 7H and 2T]
$9C7*.5^9*.5^2$

This equation doesn't yield the correct answer. What is wrong?

**Debsta** · April 17th 2010, 05:30 PM

Originally Posted by dwsmith

p[of tossing nine coins and getting 7H and 2T]
$9C7*.5^9*.5^2$

This equation doesn't yield the correct answer. What is wrong?

Why is 9 your first power of 0.5?

**dwsmith** · April 17th 2010, 05:31 PM

$nCr*.5^n*.5^{n-r}$

**Debsta** · April 17th 2010, 05:40 PM

Originally Posted by dwsmith

$nCr*.5^n*.5^{n-r}$

Check your formula!!!!

**dwsmith** · April 17th 2010, 05:40 PM

The book sucks don't have one. I had to find that online.

**Debsta** · April 17th 2010, 05:43 PM

Originally Posted by dwsmith

The book sucks don't have one. I had to find that online.

You've either gone to a dodgy site or copied it down incorrectly. Your first power of 0.5 should be r not n.

Thread: 9 coins: 7H and 2T