A 1/20 serial dilution of a bacterial culture was performed in tubes "A" through "E" starting with tube "A" and proceeding to tube "E". 100 microliters of the diluted culture from the final tube "E" was spread onto a plate. Following incubation of the plate, growth of the bacteria produced 50 colonies on that plate. What is the concentration of viable cells in tube "B" (in cells per milliliter)? (2 pts)