Homework Help: Permutation and combination problem with coins

1. Mar 2, 2012

kenny1999

1. The problem statement, all variables and given/known data

Hello, it is a permutation / combination approach question, however, having thought about an hour i can't get any idea how it should be solved.

Question is

A coin is flipped n times, where n>=3

Find the number of ways to obtain

I am not asking for answers. I have the answers but I don't understand... I don't know how to understand the problem from the definition of permutation and combination.

2. Relevant equations

3. The attempt at a solution

2. Mar 2, 2012

HallsofIvy

To get exactly two heads, you must also get n- 2 tails. How many ways can you arrange HHTTTT...T?

"At least two heads" would mean "NOT 0 heads or 1 head". There are $2^n$ ways to arrange n letters that can be "H" or "T". How many ways are there to arrange HTTTT...T?