Soil at a site consists of two layers. The top layer permeability k units and bottom layer has permeability 5k units. If the thickness of both layers is equal, than what is the average permeability in the vertical direction?

Correct option is B

​$$H_1 =H$$ , $$K_1 =K$$
$$H_2 =H$$, $$K_2 =5K$$
$$K_{Avg} = ?$$
$$K_{Avg} = \frac{H_1+H_2}{{\frac{H_1}{K_1}}+{\frac{H_2}{K_2}}}$$
 $$K_{Avg} = \frac{H+H}{{\frac{H}{K}}+{\frac{H}{5K}}}$$
$$K_{Avg} =$$ (5/3)k​​