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Calculating colony forming units?

Hope this is in the right place, if not sorry!
I'm a bit stuck on what i'm supposed to be doing in this calculation.. If anyone could give me some pointers i'd really appreciate it :smile:

"Soil suspension has been prepared as follows: 10g soil was made up to 15ml with peptone solution. You are provided with a 10^-4 dilution of the original suspension. This was prepared in two steps; 0.1ml of suspension was added to 9.9ml peptone to give 10^-2 dilution. This was repeated to give the 10^-4 dilution.
Calculate the number of colony forming units (CFUs) per gram of soil in your sample, take careful account of the dilution used for counting in your calculation"

I prepared 10^-5 10^-6 and 10^-7 dilutions by removing 0.5ml each time to the new dilution.
I plated 1ml samples of the four dilutions. Then after incubating I counted 300 colonies in the 10^-5 dilution..

Now i'm stuck on what to do to calculate per ml or g .. I know to multiple the number of colonies by the dilution factor so 300 * 10^5 .. but do I need to do something else to get the answer?
Thanks! :smile:

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