In forming a 3 digit number the 100’s place can be occupied by any 9 out of 10 digits (0 not included because it will lead to formation of 2 digit number.)
The 10’s place can be occupied by any of the remaining 9 digits (here 0 can or cannot be used.)
In one’s place any of the remain 8 digits can be used.
So total 3-digit numbers with no digit repeated are: 9 × 9 × 8 = 648.
