Co-prime Composites

a + b = c a+b=c

Let a , b , c a,b,c be positive composite integers, satisfying the equation above, such that a b c = p n # abc = p_n \# for some n t h , n^{th,} prime.

What is the least possible value of c c ?

Note : p n # p_n \# is primorial, e.g. p 5 # = 2 3 5 7 11 p_5 \# = 2•3•5•7•11 .


The answer is 231.

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1 solution

Jon Haussmann
Jul 31, 2018

We can check that a b c = p 6 abc = p_6 does not work, but for n = 7 n = 7 , 2 5 + 13 17 = 3 7 11 , 2 \cdot 5 + 13 \cdot 17 = 3 \cdot 7 \cdot 11, so the answer is 3 7 11 = 231 3 \cdot 7 \cdot 11 = 231 .

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