A company has a large of computer assistants, each of whom is competent in the use of at least one of 3 utility packages; Word processor (W) Database Management system (D) and a Spreadsheet (S). A survey shows that 30 can use a word processor, 25 can use a Database Management system and 28 are competent in the use of a spreadsheet. Of the computer facility assistants who can use a Database Management System, 14 can also use a word processor while 6 have no other skill. 6 of the computer assistants can use a word processor and spreadsheet but not a database system. 4 have all three skills. Determine the number of computer facility assistants.