Sunday Times Teaser 2812 – Sum Card Trick

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?