Answer:
3.4Ë£[tex]10^{18}[/tex] bacterial cell.
Explanation:
Generation times is 15 minutes then 4 generation we can get in 1 hour.
so, in 12 hours we can get 12Ë£4= 48 generation.
original number of cell sample was 12000
then bacteria present in 12 hours  12000ˣ[tex]2^{48}[/tex]= 3.4ˣ[tex]10^{18}[/tex].