$($a$)$ Box A weighs $300$ lb and Box B weighs $100$ lb$.$ The two boxes are connected by a cable that runs through a frictionless pulley. The coefficient of friction between Box A and the ramp is $$s $=0\mathrm{.}35.$ What is the magnitude of the friction force exerted on Box A by the ramp?$($b$)$ If Box B weighs $100$ lb and $$s $=0\mathrm{.}35,$ what is the maximum weight for Box A that would hold the system in static equilibrium?