Answer
$Q(t) =726\cdot (0.94374)^t$
Work Step by Step
The problem describes an exponential function with the following parameters.
\begin{equation}
\begin{aligned}
a&= 726\\
r&= 5.626\%= 0.05626\\
b&=1-r = 0.94374\\
\end{aligned}
\end{equation}
Hence, a model of the mass the tritium is
\begin{equation}
\begin{aligned}
Q(t)&= a\cdot b^t\\
&= 726\cdot (0.94374)^t
\end{aligned}
\end{equation}
A graph of the function is given below.
