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


So we are given with 20 \frac{rev}{min} and we want to make the unit as  \frac{rad}{s} .

In order to do that, we just have to remember that 1 revolution=2π rad

Doing the necessary conversions:
20 \frac{rev}{min} * \frac{1 min}{60 s}* \frac{2 \pi rad}{1 rev}

By doing the arithmetic you will be able to get the value of the angular velocity in rad/s.

Hope that helps.