This problem can be solved by both trial and improvement and by using simple algebra. It is the intriguing kind of "puzzle-game" that can be taken from the maths classroom into the playground! A poster of this problem is available here.
This printable worksheet may be useful: Think of Two Numbers.
You could introduce the problem as it appears on the site as a printed sheet or on a computer. Learners could first work individually to give them 'thinking time', then work in pairs to support each other and to give an opportunity for mathematical talk, and finally there could be a class discussion.
Have you tried with several numbers to see what is happening?
Suggest trying with different numbers, thus practising simple calculation, even if the generalising is not done.
Learners could go on to Multiply the Addition Square.