Mathematics Class 8
Number Play
Write a number as digits times place value. Prove tricks and test divisors.
14 lessons · ~104 min total
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Lessons in this topic
- 0 Start here A roadmap: digits, reversing, divisibility tests, and arguing for every number. · 4 min You are here
- 1 Ten of one digit, plus the other A two-digit number is 10a + b. Add it to its reverse and you always get 11(a + b). · 7 min
- 2 Three digits, reversed and subtracted A three-digit number minus its reverse is 99 times the gap between its outer digits. · 8 min
- 3 The test that alternates the signs Add and subtract the digits by turns, starting at the ones. That total tests for 11. · 8 min
- 4 A check that misses some mistakes Compare the marks of two numbers on 9. A disagreement proves a wrong answer — agreement does not prove it right. · 9 min
- 5 When a letter stands for a digit A letter stands for one digit. A B means 10A + B, so work backwards to the digits. · 8 min
- 6 Painting a test on a hundred squares Shade every multiple of a number on a 1-to-100 grid. The shape shows its step size. · 8 min
- 7 From five examples to every number Write the digits as letters, do the sum once, and read off the factor left over. · 9 min
- 8 The end of a number decides The last few digits decide for 2, 5, 10, 4 and 8. · 7 min
- 9 Add the digits to test 3 and 6 A number divides by 3 when its digit sum does. For 6, test 2 and 3. · 8 min
- 10 Two tests that make one To test 40, check 5 and 8. The pair 4 and 10 is not enough. · 8 min
- 11 Checkpoint: quick divisibility tests Check the end-digit tests, the digit-sum test and the pair for 40. · 5 min
- 12 The big picture Ten ideas, tied back together. · 6 min
- 13 Track quiz Cheat-sheet table plus a reveal quiz — one question per idea. · 9 min
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