A certain substance decomposes according to a continuous exponential decay model. To begin an experiment, the initial amount of the substance is $320kg.$ After $8$ hours, $240kg$ of the substance is left.

$($a$)$ Let $t$ be the time $($in hours$)$ since the beginning of the experiment, and let $y$ be the amount of the substance $($in $kg)$ at time $t.$

Write a formula relating $y$ to $t.$

Use exact expressions to fill in the missing parts of the formula.

Do not use approximations.

$y=e^{D}t$

$($b$)$ How much of the substance is left $9$ hours after the beginning of the experiment?

Do not round any intermediate computations, and round your answer to the nearest whole number.