Calculate dilution by taking the amount added divided by the final volume (ie. If you added 1 ml to 9 ml, then 1/1+9= 1/10 (1 ml added, 10 ml was the final volume).
Calculate total dilution by multiplying the dilutions - for example, if we diluted a sample 4 times - 1ml added to 9ml each time - then the dilution is a 1:10 of a 1:10 of a 1:10 of a 1:10. In mathematical sentences, "of" means multiply. So, 1/10 x 1/10 x 1/10 x 1/10 = 1/10,000. (notice that there are 4 dilutions of 1/10 and the final dilution has 4 zeros. Count the zeros in your problem and in your answer to check yourself.)
CFU's/ml - The CFU's are colony forming units. Since we assume that one colony is the result of 1 bacteria cell which originally was inoculated on the plate (not two or more that were very close together), we call them colony forming units.
The formula is: CFUs/ml = Total CFUs on the whole plate / amount plated x dilution 1. Determine the amount of CFUs on the whole plate. Many questions will give the amount on a 1/4th or ½ of the plate. That is not the whole plate. To estimate the amount on the whole, multiply by 4 (if 1/4th is given) or 2 (if ½ is given).
2. Determine the total dilution.
One thing to remember - if you plate 0.1ml instead of a full 1.0ml, your total dilution on the plate is 1/10 of your dilution. Unless otherwise stated, assume that 1ml of your diluted sample, was plated.
Multiply the total CFUs/plate x the inverse of the total dilution. (The dilution is a fraction. When dividing by a fraction, you are essentially multiplying by the inverse of the fraction. Therefore, CFUs x inverse of dilution is another way to look at it.)