Here are some useful facts: 1) The set of all languages (over some nonempty alphabet) is uncountable. 2) The union of two countable sets is always countable, 3) The set of Turing machines is countable and 4) Every deciable language is decided by some TM. You now have everything you need to do the following problems quickly and easily Required:
Show that the class of decidable languages is countable.