Use the half-life information to complete each table. (The formula $\mathcal{N}=\mathcal{N}_{0} e^{k t}$ is not required.)
(a) Uranium-228: half-life $=550$ seconds$$\begin{array}{llllll}t \text { (seconds) } & 0 & 550 & 1100 & 1650 & 2200 \\\mathcal{N} \text { (grams) } & 8 & & & \\\hline\end{array}$$
(b) Uranium-238: half-life $=4.9 \times 10^{9}$ years$$\begin{array}{lcccccc}\hline \multirow{2}{*}\begin{array}{l}\text { t (years) } \\\mathcal{N} \text { (grams) }\end{array} & \multicolumn{2}{c}0 \\& \multicolumn{2}{c}10 & 5 & 2.5 & 1.25 & 0.625 \\\hline\end{array}$$