7. Friction, Inclines, Systems

A skier of mass $m$ is sliding down a frictional inclined plane that makes an angle of $15°$ with the horizontal. If the coefficient of kinetic friction between the skis and the snow is $\mu$, which of the following expressions gives the skier's acceleration down the slope?

A 3-kg block is at rest on an adjustable ramp. When the ramp is tilted to a 20° angle, the block slides with constant velocity. What is the coefficient of kinetic friction between the ramp and the block?