Final answer:
The number of strings of length 4 starting with the letter B using the letters ABCDE without repetitions is 24.
Step-by-step explanation:
In this problem, we need to find the number of strings of length 4 starting with the letter B using the letters ABCDE without repetitions.
Since we are not allowed to repeat any letters, for the first letter (which is B), we have only one choice. For the second letter, we have 4 choices (A, C, D, E). For the third letter, we have 3 choices, and for the fourth letter, we have 2 choices.
Therefore, the total number of strings is 1 * 4 * 3 * 2 = 24.