-elephone numbers are preceded by a 3-digit area code (e.g. 647). In the following questions, any digit from {0,1,2,3,4,5,6,7,8,9} can be used unless otherwise stated. How many area codes are possible if: a. the second and third digits must be the same? b. the first digit cannot be 0 or 1 , the second digit cannot be 1 or 9 , and the third digit must be 9 ? c. neither 0 nor 1 must be used? d. the digit 1 must not be used? -elephone numbers are preceded by a 3-digit area code (e.g. 647). In the following questions, any digit from {0,1,2,3,4,5,6,7,8,9} can be used unless otherwise stated. How many area codes are possible if: a. the second and third digits must be the same? b. the first digit cannot be 0 or 1 , the second digit cannot be 1 or 9 , and the third digit must be 9 ? c. neither 0 nor 1 must be used? d. the digit 1 must not be used