Question:
How many three-digit numbers are there, with no digit repeated?
Solution:
Total number of 3-digit numbers $=$ Number of arrangements of 10 numbers, taken 3 at a time $={ }^{10} P_{3}=\frac{10 !}{7 !}=10 \times 9 \times 8=720$
Total number of 3-digit numbers, having 0 at its hundred's place $={ }^{9} \mathrm{P}_{2}=\frac{9 !}{7 !}=9 \times 8=72$
Total number of 3-digit numbers with distinct digits $={ }^{10} P_{3}-{ }^{9} P_{2}=720-72=648$