A large box sits on a rough floor. A person pushes on the box with a horizontal force of magnitude 50 N. The box remains at rest. What is the magnitude of the friction force

The magnitude of the frictional force is  

Explanation:

From the question we are told that

   The force exerted on the box is  

Generally for the box to remain at rest then it means that the frictional  force is greater than or equal to the force applied to move it i.e

=>      

the diagram which shows forces as either balanced = same speed or unbalanced = accelerarting