b
$\mathrm{f}=0.1 \times 4 \times \frac{10}{\mathrm{h}_{1}}+0.4 \times 4 \times \frac{10}{\mathrm{h}_{2}}$
$=\frac{0.1 \times 4 \times 10}{3-h_{2}}+\frac{0.4 \times 4 \times 10}{h_{2}}$
for $f_{\min } \frac{d f}{d h_{2}}=0$
$\mathrm{h}_{2}=2 \mathrm{m}$
$\mathrm{F}_{\min }=12 \mathrm{N}$