Question 882220
1. with repeated letters and numbers the answer will be
 
{{{26^3*10^3}}}   (that is 26 X 26 X 26 X 10 X 10 X 10)

because you can choose one of 26 letters each time and one of 10 digits each time.

2. with no repetition allowed the answer will be

{{{26*25*24*10*9*8}}}

because 

the first letter can be chosen from 26 letters, the second chosen from only 25 letters (no repeats!) and the next can be chosen from 24. the same goes for the digits..10 digits for 1st choice, 9 digits for 2nd choice, 8 digits for 3rd choice.

You could use a calculator to work out their numerical value if you wished.