Answer:
The answer is 156.5N.
This problem involves the application of newtons third law of motion and the knowledge of frictional forces. The law which states that action and reaction are equal and opposite.
Explanation:
The approach to solving this problem draws the diagram that vonnects the three crates and sections the interconnection surface as can be seen below. The force of friction acts on each of the crates and is equal to uR where R is the reaction force of the ground on each of the crates.