In this case we can use the exponential growth model, given by
[tex]P=P_0(1+r)^t[/tex]where P_0 is the inital population, r is the rate and t the time. In our case, we have
[tex]\begin{gathered} P_0=20 \\ r=0.5\text{ \lparen corresponding to 50\%\rparen} \\ t=12\text{ week} \end{gathered}[/tex]So, by substituting these values into the model, we have
[tex]P=20(1+0.5)^{12}[/tex]which gives
[tex]\begin{gathered} P=20\times129.7463 \\ P=2594.9267 \end{gathered}[/tex]Therefore, by rounding off to the nearest whole number, the answer is 2595 bacteria