A Water Tank Has Been Contaminated with Salt and Must $y=.1+3.02 e^{-.4 t}$

Multiple Choice

A water tank has been contaminated with salt and must be flushed with pure water. The concentration of salt in the tank is given by the equation $y=.1+3.02 e^{-.4 t}$ , where $y$ is the concentration of salt in milligrams per gallon and $t$ is the time, in hours, since flushing began. Match each question with one of the solutions $\mathrm{A}, \mathrm{B}, \mathrm{C}$ , or $\mathrm{D}$ .
-How long will it take for the concentration to reach half its initial value?

A) Evaluate $.1+3.02 e^{-.4(1 / 2) }$
B) Solve $\frac{1}{2}(3.12) =.1+3.02 e^{-.4 t}$
C) Evaluate $.1+3.02 e^{-.4(2) }$
D) Solve $2=.1+3.02 e^{-.4 t}$