\[
\begin{tikzpicture}
\begin{axis}[
xlabel={Time (days)},
ylabel={Mass remaining (mg)},
xmin=0, xmax=40,
ymin=0, ymax=800,
xtick={0,5,10,15,20,25,30,35,40},
ytick={0,100,200,300,400,500,600,700,800},
legend pos=north east,
grid style=dashed,
width=10cm,
height=7cm
]
\addplot[color=blue,domain=0:40,samples=100]{800*exp(-ln(2)/5*x)};
\legend{Mass remaining}
\end{axis}
\end{tikzpicture}
\]