A $3-$ft depth of stagnant water at $25$oC lies on top of a $0\mathrm{.}10-$in$.$ thickness of NaCl. At time t $<0,$ the water is pure. At time t $=0,$ the salt begins to dissolve into the water. If the concentration of salt in the water at the solid$-$liquid interface is maintained at saturation $(0\mathrm{.}00544$ mol NaCl$/$cm$3)$ and the diffusivity of NaCl is $1\mathrm{.}2\backslash$times $10-5$ cm$2/$s$,$ independent of concentration, estimate, by assuming the water to act as a semi$-$infinite medium, the time and of salt in the water when: $($a$)10\%$ of the salt has dissolved; $($b$)50\%$ of the salt has dissolved; and $($c$)90\%$ of the salt has dissolved. Density of NaCl $=2\mathrm{.}165$ g$/$cm$3.$ Molecular weight of NaCl $=58\mathrm{.}44$ g$/$mol$.$