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?
Questions like this do not appear on the current SAT, so if you’re finding this in a prep book, you may want to consider a different prep book.
To solve a problem like this, set up blanks—one for each of the four letters in a radio station call sign.
Then figure out in the number of possible ways to fill each blank. The first letter can only be a W or a K; so there are two choices for that letter.
Each of the next three letters can be ANY letter in the alphabet. There are 26 letters, and the question says repetition is allowed, so there are 26 possible fills for each of the next three blanks.
2 26 26 26
Multiply all those together to figure out the number of possible call signs.