[SPLIT] prob. of getting four 'heads' with unfair coin

Just to make sure that I am doing this right, I want to try another question.

You are using an unfair coin, the prob. of getting heads is .6, and the prob. of getting a tails is .4
The coin is tossed 7 times

The prob. of getting four heads


(6/10*6/10*6/10*6/10*4/10*4/10*4/10)n


1. My question: is n the total number of combinations?
If that's the case, then n is 21 right?

I should have written
(6/10^4*4/10*3)-makes it easier

The correct answer is \(\displaystyle \left( {\begin{array}{c}
7 \\
4 \\
\end{array}} \right)\left( {.6} \right)^4 \left( {.4} \right)^3 .\)

This is a side bar.
If “by tossing a coin” one means that the coin is spinning in air and caught on the fly (does not land on its own) then there is no such a thing as unfair coin. This was proved several years ago. Think about it: as it spins and is caught a weight would not make a difference. Physics is still arguing about allowing it to land while spinning.

alright thanks.