Here is a schematic presentation of how Combinatorics work.
Ask yourself two questions.
First, Does the order in which thing are done (or items are selected) matter in this question. If the answer is yes than its a Permutation problem. If the answer is no than it's a Combination problem.
Now when you have determined whether it's a combination or a permutation problem ask yourself the second question, is repeated selection allowed in this problem?
If "r" is the number of choices to be made and "n" is the total number of choices allowed than use the appropriate formula.
Permutation with repetition: n^r
Permutation without repetition: n!/(n-r)!
Combination with repetition: (n+r-1)/r!(n-r)!
Combination without repetition: n!/r!(n-r)!
Hope it helps, will try to add more detailed explanation with time. (Check the attachment and kindly give your feedback and suggestion).
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