\(\mathrm{F}\left(\mathrm{b}+\frac{\mathrm{a}}{2}\right)=\mathrm{mg} \frac{\mathrm{a}}{2}\dots (2)\)
\(\mu m g\left(b+\frac{a}{2}\right)=m g \times \frac{a}{2}\)
\(\left(b+\frac{a}{2}\right) \mu=\frac{a}{2}\)
\(0.4=\mu=\frac{a}{2 b+a}\)
\(0.8 b+0.4 a=a\)
\(0.8 \mathrm{b}=0.6 \mathrm{a}\)
\(\frac{b}{a}=\frac{3}{4}\)