Answer all questions with explanation

An IT system allows the users to access the system using passwords of lengths ranging from five to eight characters long with the following specifications: the length five characters can only be bit string with three consecutive 1s; the length six passwords can be case independent alphabets or bits, but each password of this length must contain one alphabet; the length seven passwords can be ACSII letters with "@" at the beginning or again "@" at the end of each password; and the passwords of length eight can be only bit strings with at most three 0s. Determine the total possible passwords are there as allowed by the IT system?