if the bacteria is tripling every 10 minutes, that means the "rate of increase" on that period is 200%, so if say the current amount is "c", 200% of "c" is just 2c, so c + 2c is 3c, a tripled amount.
[tex]\bf \textit{Periodic Exponential Growth}\\\\
A=I(1 + r)^{\frac{t}{p}}\qquad
\begin{cases}
A=\textit{accumulated amount}\\
I=\textit{initial amount}\to &1\\
r=rate\to 2\%\to \frac{200}{100}\to &2.00\\
t=\textit{elapsed time}\to &40\\
p=period\to &10
\end{cases}
\\\\\\
A=1(1 + 2)^{\frac{40}{10}}[/tex]
