IIT-JEE 1982Question 376
$mn$ squares of equal size are arranged to form a rectangle of dimension $m$ by $n$, where $m$ and $n$ are natural numbers. Two squares will be called 'neighbours' if they have exactly one common side. A natural number is written in each square such that the number written in any square is the arithmetic mean of the numbers written in its neighbouring squares. Show that this is possible only if all the numbers used are equal.
Permutation and Combination
Write your solution on paper, then reveal the worked solution to compare.