A 6 m thick clay layer undergoes 90% consolidation four times faster under two-way drainage as compared to one-drainage. In an identical clay layer of 15 m thickness, two-way drainage will be faster as compared to one-way drainage by

Time factor T_v = Cv * t / d^2. For same degree of consolidation, t is proportional to d^2. For two-way drainage, d = H/2. For one-way, d = H. Ratio of times = (H/2)^2 / H^2 = 1/4. The 15m layer is 2.5 times thicker than 6m, so time increases by 2.5^2 = 6.25. The question asks for the comparison of drainage types, which is always 4 times.