Asled weighing 300 n is moved at constant speed over a horizontal floor by a force of 50 n applied parallel to the floor
determine the coefficient of friction between the sled and the floor

If the sled is moving at a constant speed, that mean there needs to be an equal but opposite force countering the 50 N force. This opposite force is friction, and is equal to the weight times a factor:

F = n*W

Let n be the coefficient of friction.

Now you simply fill in the formula and you get:

50 N = n*300N or n = 50N / 300N = 1/6

So, the final answer is 1/6 or 0.1667