aThe radioactive isotope $\backslash (\{\}^\{239\}\backslash$mathrm$\{$Pu$\}\backslash )$ has a half$-$life of approximately $24100$ years. After $1800$ years, there are $5\mathrm{.}5$ g of $\backslash (\{\}\_\{5\}^\{5\}239\backslash$mathrm$\{$Pu$\}\backslash ).$

$($a$.)$ What was the initial quantity? Round your answer to three decimal places.

Correct Answer

$($b$.)$ How much of the isotope remains after $18000$ years? Round your answer to three decimal places.