mary purchased a package of 9 different plants but she only needed 6 plants for planting. In how many ways can she select the 6 plants from the package to be planted?
So, you have nine-element set and its cardinality n is equal to 9. You have to choose 6 elements from it. Because order of choosing doesn't have meaning, we can say that you have to compute how many subsets with cardinality 6 you can distinguish. This can be done with this formula: \[9! \over 6!(9 - 6)!\]. The result is 84
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