A steel washer is suspended inside an empty shipping crate from a light string attached to the top of the crate. The crate slides down a long ramp that is inclined at an angle of 31^\circ above the horizontal. The crate has mass 210 {\rm kg}. You are sitting inside the crate (with a flashlight); your mass is 57 {\rm kg}. As the crate is sliding down the ramp, you find the washer is at rest with respect to the crate when the string makes an angle of 57^\circ with the top of the crate.
What is the coefficient of kinetic friction between the ramp and the crate?