A stratum of soil consists of three layers of equal thickness. The permeability of both the top and the bottom layers is 10-4 cm/s and that od middle layer is 10-3 cm/s ; then the value of the horizontal coefficient of permeability for the entire composite of the soil layers is
1
2 × 10-4 cm/s
2
3 × 10-4 cm/s
3
4 × 10-4 cm/s
4
5 × 10-4cm/s