Equality of rows and columns of a table with numbers

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In a table of size $m \times n$, numbers are written so that the sum of the numbers in each row and in each column is equal to $10$.

How to prove that $m = n$?

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1 Answer

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Hints:

The sum of all the rows is equal to ________?

The sum of all the columns is equal to ________?

But those two sums have to be equal (why?)

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