Permutation and combination problem with coins

[kenny1999]

Homework Statement

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) exactly two heads
(ii) at least 2 heads

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.

Homework Equations

The Attempt at a Solution

 

[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?