Question

If I plated 0.1 mL of a 10⁻5 dilution of E. coli culture and saw 75 colonies on the plate the following day, what was the CFU/mL of the undiluted culture?
a) ( 7.5 × 10⁸ )
b) ( 7.5 × 10⁷ )
c) ( 7.5 × 10⁶ )
d) ( 7.5 × 10⁵ )