Odd or Even Sum? Decide Without Adding

Twelve additions are printed on this 3rd grade page in two columns, each one a pair of three-digit numbers with the final digit of both underlined in coral. Two circles beside every card wait under the words odd and even. A small table at the top gives three rules: even plus even is even, odd plus odd is even, even plus odd is odd. The sheet shown runs 963 plus 573, 549 plus 858, 861 plus 316 and nine more of the same shape.

The instruction says not to add, and the numbers are large enough to mean it: a child who tries to work out 963 plus 573 has missed the page. What is being taught is that parity is a property of numbers that survives addition, so the last digit of each addend is the only thing that matters and the other four digits are noise. Six of the twelve sums are even and six odd, so the table has to be used every time.

One sheet, twelve additions, twelve ticks. Note that the table lists even plus odd but not odd plus even, and three of the twelve cards present the odd number first — the point that order does not change the answer is worth making out loud. This is the top of the odd-and-even family and the only face that works above one hundred.