
Order

is important

Order is NOT important


Arrange
Permutations

Arrange and Pick
Variations

Pick
Combinations 
Without repetition 
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How many ways are there to arrange 3 letters a,b,c? _{} 
How many words of 2 different letters can you make with 4 letters a,b,c,d? _{} 
How many ways are there to pick 2 different letters out of 4 letters a,b,c,d? [number of subsets] _{} 
With
repetition 
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Example 
How many ways are there to arrange 2 letters a and 2 letters b? _{} 
How many words of 2 letters can you make with 4 letters a,b,c,d? _{} 
How many ways are there to pick 2 letters
out of 4 letters a,b,c,d? _{} 
Dirk Van de moortel – 2003