A30 kg box rests on a 15 degree incline. find fn.

N = normal force by the surface of incline on the box resting on it

m = mass of the box resting on the incline surface = 30 kg

g = acceleration due to gravity acting on the box = 9.8 m/s²

θ = angle of the incline with the horizontal surface = 15⁰

from the force diagram , force equation perpendicular to incline surface is given as

N = m g Cosθ

inserting the values in the above equation

N = (30 kg) (9.8 m/s²) Cos15

N = 284 newton

if you already have the answer to (a), then just work out how many periods there are in 1 minute, and that's the answer to (c).