Study Guide · Chapter 23
2.21 Quick Reference Table
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Figure: The nine most-used divisibility rules, at a glance.
| Divisor | Rule (test on the number) |
|---|---|
| 2 | Last digit even |
| 3 | Digit sum divisible by 3 |
| 4 | Last 2 digits divisible by 4 |
| 5 | Last digit 0 or 5 |
| 6 | Divisible by 2 and 3 |
| 7 | Double last digit, subtract from rest, repeat |
| 8 | Last 3 digits divisible by 8 |
| 9 | Digit sum divisible by 9 |
| 10 | Last digit 0 |
| 11 | Alternating digit sum divisible by 11 |
| 12 | Divisible by 4 and 3 |
| 13 | 4×last digit, add to rest, repeat |
| 14 | Divisible by 2 and 7 |
| 15 | Divisible by 3 and 5 |
| 16 | Last 4 digits divisible by 16 |
| 17 | 5×last digit, subtract from rest, repeat |
| 18 | Divisible by 2 and 9 |
| 19 | 2×last digit, add to rest, repeat |
| 25 | Last 2 digits ∈ {00,25,50,75} |
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