Find the average angular velocity (in rad/s) of a wheel of radius 30.0 cm that starts from rest and makes 6.00 revolutions in 2.50 s. this is my work, but the answer i am getting is not correct: w avg = angular displacement / time = 6.00 rev * (2π rad / 1 rev) / (2.50 s) = 15.079644 rad / s = 15.1 rad/s

As I waited for the question to open so I could answer it, I went through
in my mind what I was planning to write.

It was exactly what you have written, starting with

                                    w avg = angular displacement / time.

I'll go out on a limb here:  You don't say where the correct answer appears. 
As far as I am concerned, your answer is correct, and if the answer key, or
the odd-numbered problems in the back of the book, or the teacher's work,
or one of the multiple choices, doesn't match yours, then it is they who have
messed up this time.  Those things are not infallible, you know. 

You have my vote.