A Sum of Odd Primes in Three Ways

Number Theory Level pending

The Goldbach conjecture states that every even integer greater than 2 can be written as a sum of two odd prime numbers. For example 12 = 7 + 5 12 = 7+5 . Some even integers can be written as the sum of two odd prime numbers in more than one way. For example 22 = 3 + 19 = 5 + 17 22 = 3 + 19 = 5 + 17 . Find the sum of all even integers less than or equal to 30 that can be written as a sum of two odd primes in exactly three ways.

98 100 102 104

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1 solution

Peter Chiappini Staff
Jul 27, 2016

The even numbers less than or equal to 30 that can be written as a sum of two odd primes in exactly three ways are 22 = 3 + 19 = 5 + 17 = 11 + 11 24 = 5 + 19 = 7 + 17 = 11 + 13 26 = 3 + 23 = 7 + 19 = 13 + 13 30 = 7 + 23 = 11 + 19 = 13 + 17 \begin{aligned} 22 &= 3 + 19 = 5 + 17 = 11 + 11 \\ 24 &= 5 + 19 = 7 + 17 = 11 + 13 \\ 26 &= 3 + 23 = 7 + 19 = 13 + 13 \\ 30 &= 7 + 23 = 11 + 19 = 13 + 17 \\ \end{aligned} Adding these we get 102.

How do you know that the other integers, (2,4,6,8,10,12,14,16,18,20,28) does not satisfy this property? By trial and error?

Pi Han Goh - 4 years, 10 months ago

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