Answer
200 N. Yes. The reaction to your push on the crate is the crate pushing back on you.
Work Step by Step
This situation is discussed on page 60.
The horizontal force of friction on the crate is 200 N, equal and opposite to your own push. There is zero net force, which explains the crate's constant velocity (zero acceleration).
The frictional force is directed opposite to your pushing force.
These two forces ARE NOT part of an action-reaction pair of forces. That’s because both forces act on the same object, the crate, and action-reaction pairs ALWAYS act on different objects.
Newton's third law says that whenever object A exerts a force on object B, we find that object B exerts an equal and opposite force on object A.
Using the law, the reaction to your push on the crate is the crate pushing back on you.
Incidentally, the reaction to the backward frictional force of the floor on the crate is the forward force of the crate on the floor.
