Pigeonhole Puzzles part 1

Pigeonhole puzzles are fun puzzles that teach addition facts, modulo 9 arithmetic, and logical thinking. They also introduce mathematical proof. They come in a range of difficulty from quite easy to quite challenging. Ideally, trial and error is not used. Instead, a digit should not be placed in a "pigeonhole" until every other possibility has been eliminated. A solution is therefore proved to be unique.

PREVIOUS LESSONS:
Teach Adding and Subtracting...

Bowling Pin Puzzles part 1...

The Harmonic Mean part 2...

The Harmonic Mean part 1...

Completing The Square Backwards...

The Quadratic Formula and a Formula for All Pythagorean Triangles...

Mathematics for Social Justice by Marc Roth.