A stratum of soil consists of three layers of equal thickness. The permeabilities of top, middle and bottom layers are k1 = 10-4 cm/sec, k2 = 2 × 10-4 cm/sec, k3 = 1.5 × 10-4 cm/sec. Then the value of average k when flow perpendicular to the bedding plane is
1
1.75 × 10-4 cm/sec
2
1.5 × 10-4 cm/sec
3
10-4 cm/sec
4
1.38 × 10-4 cm/sec