A 15.0 kg block is pulled by a horizontal force of 30.0 N along a rough horizontal surface at constant velocity. What is the coefficient of friction?

Because speed is constant that means that friction force is equal to the force that is pulling the object.

According to Newtons law, if resultant speed is not equal to 0, object will accelerate. In this case object is not accelerating which means that resultant force is indeed equal to 0

Because only forces that are affect the block are:

1 Pulling force

2 Friction force

that means that they have to be equal

Answer is 30N