d
\(P _{ i }= P _{ f }(\text { Collision })\)

\(\Rightarrow(0.1)(400)=(0.1+3.9) v\)

\(\Rightarrow v =\frac{0.1 \times 400}{4}=10\,m / s\)

\(a =\frac{\mu mg }{ m }=\mu g\)

Apply equation of motion,

\(v ^2=u^2+2 as\)

\(\Rightarrow 0=(10)^2-2 \mu g \times 20\)

\(\Rightarrow 40 \mu g =100\)

\(\Rightarrow \mu=\frac{100}{2 \times 10 \times 20}=\frac{1}{4}\)