From Historical Connections in Mathematics, Volume II, page 85
Many children we have taught enjoy making lists, and kids who are not strong at their number facts show enthusiasm for solving this problem.
Find all the equations of the form __ + __ = __ using only the numbers 1 - 9 in each blank, and using each number only once in each equation, e.g., 1 + 2 = 3.
1 2 3
4 5 6
7 8 9
Once you think you have found all the equations, how can you check that you haven't forgotten any?
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