Permutation and Combination

Permutation and Combination are used to find how many possible outcomes.

Permutation is used when order is important

Combination is used when order is unimportant

the formulas are:

nPr = n!/(n-r)!

nCr = n!/(n-r)!r!

! is the factorial symbol which means multiplied by every number less than that number all the way to one. so like 5! would be 5*4*3*2*1

0! = 1

Examples:

1. You have a pile of 5 different looking coins. How many ways can you take 2 coins from the pile?

since the coins are different, order is important and we use permutation

5P2= 5!/(5-2)!

5!/3! =20

2. Twelve people apply for three jobs, if the jobs are all the same how many ways can three people been picked?

since the jobs are all the same, order is unimportant and we use combination

12C3= 12!/(12-3)!3!

12!/9!3!= 1320/3!

220

thats how it is done.