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You have 40 CFU on a plate on which you plated 0.125 mL. You are told...

You have 40 CFU on a plate on which you plated 0.125 mL. You are told the original sample was diluted 1 mL into 11 mL. Next, 0.5 mL of this was diluted into 49.5 mL and 2 mL of this was diluted into 18 mL. The 40 colonies came from the last dilution, what was the original CFU mL-1?
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These questions are solved by finding the total dilution performed and any volumetric correction required to then find out the concentration in the original sample. In this example, the following image gives the different dilution in each step and the total dilution till that step. The CFU count in the original sample is 3.84 * 106 CFU/mL.

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