We usually write numbers in decimal form (or base 10), meaning numbers are composed using 10 different “digits" {0,1, ...,9}. Sometimes though it is useful to write numbers hexadecimal or base 16. Now...

Extracted text: We usually write numbers in decimal form (or base 10), meaning numbers are composed using 10 different “digits" {0,1, ...,9}. Sometimes though it is useful to write numbers hexadecimal or base 16. Now there are 16 distinct digits that can be used to form numbers: {0, 1,...,9, A, B, C, D, E, F}. So for example, a 3 digit hexadecimal number might be 2B8. Assume that digits and letter can be repeated. a. How many 4-digit hexadecimals are there in which the first digit is E or F? b. How many 5-digit hexadecimals start with a letter (A-F) and end with a numeral (0-9)?