Therfore 18Case c: The 3 letter code will contain both A and B but not togatherFirst Letter A , 2nd any letter other than A or B, Third Letter B1x3x1 = 3But The first letter can be either A or B. Therefore we have 2 cases 3x2=6Total no of ways = 18 + 18+ 6= 42 There is one more case, depending on how you read it Case [d].