how many different 6-digit serial numbers are available if no diget can be repeated?

Answer 1

there are 10 possible numbers for the first digit (1,2,3,4,5,6,7,8,9,0), then 9 for the second, 8 for the third, 7 for the fourth, 6 for the fifth, and 5 for the sixth.
10*9*8*7*6*5 = 151200 possible serial numbers

Answer 2

the first number can be placed in 6 different places the second 5 different the third 4 different plces the fourth 3 different places the fifth 2 different and the last only 1 possible, so 6*5*4*3*2*1

Answer 3

If you are based out of the fact that there are 10 digits, 0,1,2,3,4,5,6,7,8,9, then what you would do is the following:

You have 10 possibilities for the first number,
9 possibilities for the second
8 for the third
7 for the fourth
and so on.

Once you have those six numbers multiply them all together and that will be your answer.

Hope this helps.