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2024 AMC 10A Problem 3

Problem 3 of 25EasierAlgebraNumber Theory

What is the sum of the digits of the smallest prime that can be written as a sum of 55 distinct primes?

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Solution

Suppose 22 is one of the five primes. Then the total is even and bigger than 2,2, so it is composite. Thus all five primes must be odd. The five smallest odd primes give 3+5+7+11+13=393 + 5 + 7 + 11 + 13 = 39 =313,= 3 \cdot 13, which is not prime. The next possible sum is obtained by replacing 1313 with the next prime, 17;17; changing any earlier term forces at least as large an increase to keep the primes distinct. This gives 3+5+7+11+17=43,3 + 5 + 7 + 11 + 17 = 43, which is prime. Its digit sum is 4+3=7.4 + 3 = 7. Thus, B is the correct answer.

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Concepts: prime · parity · optimization

Problem text and solution from the LIVE past-contest archive. See also the AoPS wiki page for community solutions.