An overnight culture of 15 ml Escherichia coli was used as a sample. Two ml of this culture is added to a bottle containing 48ml of buffer. This dilution is mixed wee (as all dilutions are!), and 5 ml of this mixed in 15 ml of buffer. This second dilution is diluted by three successive 1:10 dilutions. the last (fifth) dilution is plated, i.e., 0.3 ml is plated on nutrient agar. after incubating the plate, 121 colonies are counted.
a) what was the final dilution in the second tube?
b) what was the final dilution in the fourth tube?
c) How many colony-forming units of E.coli culture were present in the original stock culture?