To start, find your equation, n=3000(.5)^t, n being your number of bacteria and t being your time. Now plug in 375 for n and solve for t, 375=3000(.5)^t, .125=.5^t, 
. This means your answer is 3 minutes. To check it, plug it into your equation. n=3000(.5)^t=3000(.125)=375