A block is kept on a rough horizontal surface. The acceleration of the block increases from $6 \mathrm{~ms}^{-2}$ to $11 \mathrm{~ms}^{-2}$ when the horizontal force acting on it increases from 20 N to 30 N . The coefficient of kinetic friction between the block and the surface is

(acceleration due to gravity $=10 \mathrm{~ms}^{-2}$ )