Help me out on permutation and combination I have to get all combinations of a six digit number where each digit is unique. Its clear that this combination will not have a 0 at the start as it will not be a valid six digit number. Help me on how to attack this problem. 
 A: First, if you're talking about a six-digit number, the order of the digits matters, so it's generally not referred to as a combination.
Consider how many possibilities there are for the first digit.  As you said, it cannot be zero, or you wouldn't have a six-digit number.  Once there is a first digit, how many possibilities are there for the second digit?  Then for the third?  And so on.
The total number of six-digit numbers is the product of the various numbers of possibilities for each digit.
(As the question feels like homework to me, this is more of a hint/outline than a complete answer.  If it actually is homework, please add the homework tag.  If it's not homework, or if this doesn't get you to an answer, please comment with more information.)
A: This seems simple : The digits will be from the set {0,1,2,3,4,5,6,7,8,9} 
Number of possibility for first digit (Most significant digit) = 9 (since we can't use 0)
Number of possibility for second digit = 9 
Number of possibility for third digit  = 8
...
Number of possibility for sixth digit (Least significant digit)  = 5
Hence total possibility = 9*9*8*7*6*5.
