5×4×3×2×1 120 1
- = = ---
8×5×4×3×2 960 8
7×6×5×4 840 1 8 9 3
+ ⅔ = + ⅔ = + --- = = ---
12×7×6×5 2520 12 12 12 4
11×10×9 7×6 4 990 42 4
- + + = - + + =
15×11×10 9×7 15 1650 63 15
3 2 4 9 10 4 5
- --- + --- + = - + + = = ⅓
5 3 15 15 15 15 15
