A bullet of mass $0.1 \mathrm{~kg}$ moving horizontally with speed $400 \mathrm{~ms}^{-1}$ hits a wooden block of mass $3.9 \mathrm{~kg}$ kept on a horizontal rough surface. The bullet gets embedded into the block and moves $20 \mathrm{~m}$ before coming to rest. The coefficient of friction between the block and the surface is __________.

(Given $g=10 \mathrm{~m} / \mathrm{s}^{2}$ )