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by Brian Gladman on October 26, 2019

New Scientist Enigma 987 – Two Halves Make a Whole

by Richard England

From New Scientist #2142, 11th July 1998

A semi-prime is the product of two prime numbers; the square of a prime counts as a semi-prime.

Harry, Tom and I were trying to find pairs of 2-digit semi-primes such that if we added the two semi-primes together we formed a 2-digit prime. We each found three such pairs; the 18 semi-primes we used and the 9 primes that were formed were all different.

Harry’s three odd semi-primes were all greater than 50; Tom’s three even semi-primes were all greater than 50.

What were my three pairs of semi-primes?

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