"How many different sums of money exist if we can choose 4 coins from any number of pennies, nickels, dimes, and quarters?" is a Combination with repetition problem.
What is combination with repetition?
A combination with repetition involves a multiset. A multiset allows elements to be repeated unlike an ordinary set.
Number of ways to select r things from n possibilities: C (n + r − 1, r)
Since he can choose any coin any number of time, repetition is there. Also, order in which coins are chosen is not important, thus it is a combination.
Therefore, the third option is correct. This is a combination with repetition problem.
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