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A number plate reads 742. Written backwards, it reads 247. That is its reverse. And 742 − 247 = 495.

This is Part I, Chapter 5, Number Play.

What you will learn

• The general form — a number as digits times place value.
• Three digits reversed — the middle digit drops out.
• The 11 test — plus and minus down the digits.
• The nine-check — a quick check that can miss a wrong answer.
• Letters for digits — find the digits that make a sum true.
• A grid pattern — a test's step drawn as a slanting line.
• Saying why — one argument that covers every number.
• End digits — the last digits decide 2, 4, 5, 8 and 10.
• Digit sums — the digits decide 3. With the test for 2, they decide 6.
• Two tests that make one — 5 and 8 give 40.

Every lesson has a lab.

Quick review: digit sums, odd and even, and checking an answer.