Since we are just being asked about giving the angular velocity (ω) of the rotating wheel in rad/s, we do not need to take into account the diameter of the wheel.
We only do that when the linear velocity is being asked. v=rω
Anyway.
So we are given with [tex]20 \frac{rev}{min} [/tex] and we want to make the unit as [tex] \frac{rad}{s} [/tex].
In order to do that, we just have to remember that 1 revolution=2π rad
Doing the necessary conversions:
[tex]20 \frac{rev}{min} * \frac{1 min}{60 s}* \frac{2 \pi rad}{1 rev} [/tex]
By doing the arithmetic you will be able to get the value of the angular velocity in rad/s.
Hope that helps.
