Hi, there-- The Problem: How many different 4 letter radio station call letters can be made if the first letter must be K or W, repeats are allowed, but the call letters cannot end in an O? . A Solution: In combinatorics, the basic counting principle states that if we have m ways of doing something and n ways of doing another thing, then there are m · n ways of performing both actions. We can use this idea to solve your radio station problem. There are 2 ways to choose the first letter of the radios call sign; it must be either K or W. There are 26 ways to choose the second letter because it can be any letter (repeats are OK.) There are 26 ways to choose the third letter. There are 25 ways to choose the fourth letter because it can be any letter but O. . Using the basic counting principle, we multiply, 2*26*26*25 = 33800 There are 33,800 ways to choose the call letters. Feel free to email me if you have questions about the solution. Ms.Figgy math.in.the.vortex@gmail.com