How many 3 digit numbers can be formed from the digits 0-9 if the digits cannot be repeated?
+9466 These confuse me too.. but let's just start writing out all the possibilities and then maybe we can see what the answer will be.
First digit 0:
01 02 03 04 05 06 07 08 09
012 021 031 041 There will also be exactly
013 023 032 042 eight possibilities under each
014 024 034 043 of these.
015 025 035 045
016 026 036 046
017 027 037 047
018 028 038 048
019 029 039 049
Iff the first digit is 0, there are 8 * 9 possibilities.
If the first digit is 1, there are also 8 * 9 possibilities.
There are 10 possible first digits, so there are 10 * (8 * 9) = 720 possibilities.
You can also just use this formula: \(_nP_r=\frac{n!}{(n-r)!}\)
Where n = 10 and r = 3
Here's a really good explanation of that formula: 
