Example $7\mathrm{.}7$: The void ratio of clay A decreased from $0\mathrm{.}572$ to $0\mathrm{.}505$ under a change in pres$-$

sure from $120$ to $180k\frac{g}{m^2}.$ The void ratio of clay $B$ decreased from $0\mathrm{.}612$ to $0\mathrm{.}597$ under the

same increment of pressure. The thickness of sample A was $1\mathrm{.}5$ times that of $B.$ Nevertheless

the time required for $50\%$ consolidation was three times longer for sample $B$ than for sample

A$.$ What is the ratio of the coefficient of permeability of $A$ to that of $B?$