How many strings of six lowercase letters from the english alphabet contain a) the letter a? b) the letters a and b? c) the letters a and b in consecutive positions with a preceding b, with all the letters distinct? d) the letters a and b, where a is somewhere to the left of b in the string, with all the letters distinct? rosen, kenneth. discrete mathematics and its applications (p. 436). mcgraw-hill higher education. kindle edition.
The letters a and b becasue those are the first two letters