Difference between revisions of "2019 AMC 10B Problems/Problem 2"
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Since a counterexample must be when n is not prime, n must be composite, so we eliminate A and C. Now we subtract 2 from the remaining answer choices, and we see that the only time <math>n-2</math> is <math>\textbf{not}</math> prime is when <math>n = 27</math>, which is <math>\fbox {E}</math>. | Since a counterexample must be when n is not prime, n must be composite, so we eliminate A and C. Now we subtract 2 from the remaining answer choices, and we see that the only time <math>n-2</math> is <math>\textbf{not}</math> prime is when <math>n = 27</math>, which is <math>\fbox {E}</math>. | ||
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==See Also== | ==See Also== |
Revision as of 11:13, 15 February 2019
- The following problem is from both the 2019 AMC 10B #2 and 2019 AMC 12B #2, so both problems redirect to this page.
Problem
Consider the statement, "If is not prime, then is prime." Which of the following values of is a counterexample to this statement?
Solution
Since a counterexample must be when n is not prime, n must be composite, so we eliminate A and C. Now we subtract 2 from the remaining answer choices, and we see that the only time is prime is when , which is .
See Also
2019 AMC 10B (Problems • Answer Key • Resources) | ||
Preceded by Problem 1 |
Followed by Problem 3 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 | ||
All AMC 10 Problems and Solutions |
2019 AMC 12B (Problems • Answer Key • Resources) | |
Preceded by Problem 1 |
Followed by Problem 3 |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 | |
All AMC 12 Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.