Question

A wheel of radius 0.5 m rotates at 400 rpm. Calculate its angular velocity.

Answer 1

1 revolution = 2π radians
Therefore
400 revolution per minute = 800π radians per minute
Angular velocity is measured in radians/time

So angular velocity is 800π rad/min

Answer 2

400 rpm is already the angular velocity!
What you have to do is probably express it in a different unit, such as rad/s for instance.