Mimi,
If all the surfaces are horizontal, which they are assuming, they are pretty simple once you understand how friction is calculated, the friction force, ff , is the product of the coefficient of friction, µ, and the normal force, Fn
Fn is the equal but opposite force to the weight of the object, mg, pushing up from the horizontal surface. So
Fn = mg.
38) since ff =µFn & Fn = weight; ff = 0.25 x 20 = 5N
39) µ =ff ⁄Fn ; µ = 12/20 = 0.60
40) ff = µFn = µmg = 0.40 x 3.2 x 9.8 = 12.5N
I let you solve 41 & 42, hope this helps
