2018 AMC 12B Problems/Problem 5
Contents
Problem
How many subsets of contain at least one prime number?
Solution 1
Since an element of a subset is either in or out, the total number of subsets of the 8 element set is . However, since we are only concerned about the subsets with at least 1 prime in it, we can use complementary counting to count the subsets without a prime and subtract that from the total. Because there are 4 non-primes, there are
subsets with at least 1 prime so the answer is
(Giraffefun)
Solution 2
We can construct our subset by choosing which primes are included and which composites are included. There are ways to select the primes (total subsets minus the empty set) and
ways to select the composites. Thus, there are
ways to choose a subset of the eight numbers, or
.
See Also
2018 AMC 12B (Problems • Answer Key • Resources) | |
Preceded by Problem 4 |
Followed by Problem 6 |
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