Problem 1 · 2020 Math Kangaroo
Easy
Number Theory
primesprime-test
Which of the following additions does not give a prime number?
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Answer: E — 5 + 7
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Hint 1 of 2
Work out each sum first, then ask which total has a factor other than 1 and itself.
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Hint 2 of 2
A number is prime only if nothing besides 1 and itself divides it — check the five totals.
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Approach: add each pair, then test the five sums for primality
- The sums are 13, 11, 17, 7 and 12.
- 13, 11, 17 and 7 are all prime (no smaller divisor works).
- 5 + 7 = 12 = 2 × 6, which is composite.
- So the sum that is not prime is choice E.
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