Example 21. How many three-digit numbers can be formed using $0,1,2,3,4,5$ without repeating any digit?
Solution (Method 1 - "Take all and subtract the unwanted"): $\mathrm{P}_{\mathrm{G}}^{\mathrm{s}}-\mathrm{P}_{5}^{2}=100$ (items).
(Solution 2 - "Take only the wanted"): $5 \mathrm{P}_{5}^{2}=100$ (items).