When using polar coordinates, there are two velocities... tangential velocity and radial velocity...
The tangential velocity is perpendicular to the radius from the origin to the point of interest...
So in this case the radius from the center of the Ferris wheel to the passenger... the tangential velocity is perpendicular to that in the direction of rotation...
So in this case the only speed is the tangential speed... there is no radial speed here... in other words the passenger goes in a circular path... the radius is fixed...
If the Ferris wheel was expanding getting a larger radius while rotating... then there'd be a radial speed to consider...
Just use distance/time to get the tangential speed...