Question:
How many three-digit numbers are there?
Solution:
Available digits for filling any place = {1, 2, 3, 4, 5, 6, 7, 8, 9, 0}
Since the thousand's place cannot be zero, available digits to fill the thousand's place = 9
Number of ways of filling the ten's digit = 10
Similarly, number of ways of filling the unit's digit = 10
$\therefore$ Total number of three digit numbers $=9 \times 10 \times 10=900$