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Answer: 13 + 15 = 28
Rows should all be the same
$\therefore$ all 34
$\therefore$ second and bottom each need to change by 2
Either swap 10, 12,
or swap 13, 15
Columns should also all be 34
$\therefore$ second column sum needs to increase
Swap 13 and 15
13 + 15 = 28
This problem is taken from the UKMT Mathematical Challenges.
You can find more short problems, arranged by curriculum topic, in our short problems collection.