# Math Help - 9 coins: 7H and 2T

1. ## 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?

2. 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?

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

4. Originally Posted by dwsmith
$nCr*.5^n*.5^{n-r}$
Check your formula!!!!

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

6. 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.