I am confused on Question 3, page 189…

I am confused on Question 3, page 189 (the topic is Measures of Central Tendency and Variability). I checked the solution and how you formed the equation

87m + 93 = 89(m+1)

But I still do not understand why 89(m+1) is there.

The call signs of radio and television stations in the United States generally begin with the letter W east of the Mississippi River and the letter K west of the Mississippi…

The call signs of radio and television stations in the United States generally begin with the letter W east of the Mississippi River and the letter K west of the Mississippi. Repetition of letters is allowed. How many different call signs are possible if each station uses a W or K followed by 3 letters?