Question

An identification code is to consist of six letters followed by there digits. How many different codes are possible if repetition is permitted?

Answer 1

6 letters with 26 choices for each letter. counting principle gives you \(26^6\) possibilities for the letters
then 3 digits with 10 choices for each digit. again counting principle give you \(10^3\) choices for the digits

taken together, one more time counting principle tell you there are \(26^6\times 10^3\) possibilities altogether.

i would use a calculator