Answer:
Ethan's tangential speed is twice as Rebecca's.
Step-by-step explanation:
Let
be the distance from the center of the platform to Rebecca's horse,
Rebecca's tangential speed,
the Ethan's tangential speed and
the merry-go-round angular speed. The distance from the center of the platform to Ethan's horse is
. The angular speed is related to the tangential speed by the equation:

Since the angular speed is the same for Ethan and Rebecca, we have that:

Now, solving for
we get:

It means that Ethan's tangential speed is twice as Rebecca's.