# 9 coins: 7H and 2T

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• Apr 17th 2010, 06:27 PM
dwsmith
9 coins: 7H and 2T
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?
• Apr 17th 2010, 06:30 PM
Debsta
Quote:

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?
• Apr 17th 2010, 06:31 PM
dwsmith
$nCr*.5^n*.5^{n-r}$
• Apr 17th 2010, 06:40 PM
Debsta
Quote:

Originally Posted by dwsmith
$nCr*.5^n*.5^{n-r}$

Check your formula!!!!
• Apr 17th 2010, 06:40 PM
dwsmith
The book sucks don't have one. I had to find that online.
• Apr 17th 2010, 06:43 PM
Debsta
Quote:

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.