Nonuniform circular motion merry-go-round

[AvrGang]

Homework Statement

A 5.0-m-diameter merry-go-round is initially turning with a 4.0 s period. It slows down and stops in 20 s.
a. Before slowing, what is the speed of a child on the rim?
b. How many revoluotions does the merry-go-round make as it stops?
( Can i know what is a merry-go-round)

Homework Equations

The Attempt at a Solution

I solved a) like that:

V= 2(Pi)r/T
V= 2(pi)(2.5)/4
V= 3.93 m/s

b) I think this is the correct equation:
(\theta)f = \thetao + \omegao t + a/2r ( t^2)

but i can't find \omegao or a

 

[Microzero]

http://www.worldsfinestshows.com/equipment/images/MerryGoRound_1.jpg

 

[Microzero]

initially turning with a 4.0 s period ---> T
ω=2pi/T
α = △ω/△T

θf = θo + ωo t + α/2( t^2)