Radioactive decay can be described by the following equation

$lnA=ln{A}_0-kt$

where $A_0$ is the original amount of the substance, $A$ is the amount of the substance remaining after time $t,$ and $k$ is a constant that is characteristic of the substance.

For the radioactive isotope iodine$-123,k$ is $5\mathrm{.}21{10}^{-2}$ hours ${?}^{-1}.$

If the original amount of iodine$-123$ in a sample is $34\mathrm{.}7mg,$ how much iodine$-123$ remains after $12\mathrm{.}4$ hours have passed?

mgM