$12\mathrm{.}18$ Block A has a mass of $40$ kg $,$ and block $B$ has a mass of $8$ kg $.$ The

coefficients of friction between all surfaces of contact are ${}_s=0\mathrm{.}20$ and

${}_k=0\mathrm{.}15.$ If $P=0,$ determine $(a)$ the acceleration of block $B,(b)$ the

tension in the cord.