[tex]\bf Q=39.2(0.732)^t\implies Q=\stackrel{\stackrel{a}{\downarrow }}{39.2}(\underset{\underset{b}{\uparrow }}{0.732})^t\implies Q=39.2(1-\stackrel{\stackrel{r}{\downarrow }}{\underset{\underset{-26.8\%}{\uparrow }}{0.268}})^t[/tex]
if the number in the parentheses is less than 1, is not growth but Decay, thus a negative rate, or namely is decreasing by that much per cycle.