Solution: We seek the largest integer $ N $ such that $ 20 < N < 40 $, and $ N $ is the sum of distinct prime numbers. - RTA
Optimal Prime Sum: Finding the Largest Integer $ N $ Between 20 and 40 Using Distinct Primes
Optimal Prime Sum: Finding the Largest Integer $ N $ Between 20 and 40 Using Distinct Primes
When tasked with identifying the largest integer $ N $ such that $ 20 < N < 40 $, and $ N $ is expressible as the sum of distinct prime numbers, prime mathematicians and puzzle enthusiasts turn to the strategy of combining prime numbers efficiently. In this article, we explore the solution method, verify all candidates, and reveal how 37 emerges as the largest valid $ N $.
Understanding the Context
What Does It Mean for $ N $ to Be the Sum of Distinct Primes?
A sum of distinct primes means selecting one or more prime numbers from the set of primes greater than 2 (since 2 is the smallest and only even prime), ensuring no prime number is used more than once in any combination. Our goal: maximize $ N $ under 40, strictly greater than 20.
Step 1: Identify Prime Numbers Less Than 40
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Key Insights
First, list all prime numbers below 40:
2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37
Note: Since the sum must exceed 20 and be less than 40, and 2 is the smallest prime, using it helps reach higher totals efficiently.
Step 2: Strategy for Maximizing $ N $
To maximize $ N $, we should prioritize larger primes under 40, but always ensure they are distinct and their total lies between 20 and 40.
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Because larger primes contribute more per piece, start with the largest primes less than 40 and work downward.
Step 3: Try Combinations Starting from the Top
We search for the largest $ N $ via targeted combinations.
Try: 37
Can 37 be written as a sum of distinct primes?
- Try with 37 itself: $ 37 $ → valid! It is prime, so $ N = 37 $
- But can we get higher? 37 is less than 40 and greater than 20 — but 38, 39, 40 are invalid (37 is the largest prime, 38, 39, 40 composite).
- Is 37 the maximum? Not yet — let's verify if 38, 39, or 40 (though invalid) can be formed — they can’t, so 37 is a candidate.
But wait — can we exceed 37 using combinations?
Try $ 31 + 7 = 38 $ → valid primes, distinct: $ 31, 7 $ → sum = 38
Try $ 31 + 5 + 3 + 2 = 41 $ → too big
Try $ 29 + 7 + 3 = 39 $ → valid
Try $ 29 + 7 + 5 = 41 $ → too big
Try $ 29 + 5 + 3 + 2 = 39 $ → valid
Try $ 23 + 11 + 3 + 2 = 39 $ — also valid
Now try $ 31 + 5 + 3 + 2 = 41 $ — too large
Try $ 29 + 7 + 3 = 39 $ — valid
Now try $ 37 + 2 = 39 $ — valid, but sum = 39
Try $ 37 + 3 = 40 $ — but 37 + 3 = 40, and 40 is allowed? Wait:
Is 40 expressible as sum of distinct primes?
37 + 3 = 40 — yes! Both primes are distinct primes.
So $ N = 40 $, but wait — the problem requires $ N < 40 $. So 40 is invalid.
