"All of the answers are adequate." and "All the answers are adequate." The above statement is incorrect. You should say: "Any answer will do." Apart from that, without the context I can't really help. If you're talking about multiple choice then: "All answers are correct." Usually we say: "All the answers." "All of them." However, you can also say: "All of the answers." Tough, huh?

the "of" is unneccessary and should be dropped as you are referring to a plural noun (i.e. answers)... a rule of thumb, I guess. :)

You can use both

The guy should have said: All (of) the answers WERE useful.