It is always a daunting task for most studying probability or mathematical statistics to figure out the number of ways a selection can be made from a set of objects. The factors which complicate the situation include the restrictions on whether the order of a selection matters, and the cases when an object from the list is allowed to be selected more than once. Permutations and combinations are two indispensable tools to carry out mathematical calculations. However, often times they can easily get mixed up. In this brief article, we review the definitions of permutations and combinations and elucidate how they can be applied in various situations to determine the count of different ways of making a selection.
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