Permutation and combination problem with coins

  • Thread starter kenny1999
  • Start date
  • #1
163
2

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

 

Answers and Replies

  • #2
HallsofIvy
Science Advisor
Homework Helper
41,833
956
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 [itex]2^n[/itex] ways to arrange n letters that can be "H" or "T". How many ways are there to arrange HTTTT...T?
 

Related Threads on Permutation and combination problem with coins

  • Last Post
Replies
2
Views
1K
Replies
2
Views
2K
  • Last Post
Replies
5
Views
2K
  • Last Post
Replies
14
Views
2K
  • Last Post
Replies
4
Views
1K
  • Last Post
Replies
5
Views
6K
  • Last Post
Replies
6
Views
17K
Replies
3
Views
828
  • Last Post
Replies
3
Views
1K
  • Last Post
Replies
1
Views
2K
Top