by Bob Walker
Published: 14 August 2016 (link)
Joe has nine cards, white on one side and grey on the other, with a single digit written on each side of each card. He gave them to Penny to arrange white-side-up to form three 3-figure numbers with the sum of the first two equaling the third. This was her attempt:
2 1 9 6 5 4 8 7 3
Then Joe turned the cards over one at a time to reveal
9 8 7 1 4 2 3 6 5
where the total of the first two did not equal the third. So he challenged Penny to arrange the cards so that both white-side-up and grey-side-up the third number was the sum of the first two, which she did.
What was her third grey number?