Answer: There are 35152 different call letters are possible.
Explanation:
Given : The call letters for all radio stations in the United States start with either a W (east of the Mississippi River) or a K (west of the Mississippi River) followed by three other letters that can be repeated.
i.e.The total characters = 4
The choices for first letter = 2 (either W or K)
The choices for rest of other letters = 26 (English alphabets has 26 letters)
Then, the number of different call letters are possible =2 x 26 x 26 x 26
=35152
Therefore , there are 35152 different call letters are possible.