If accuracy is not a problem the most simple way to produce 20 ml of a 1:400 dilution is to take 1:400 of 20 ml (i.e. 0.05 ml) of the original solution and add 19.95 ml of liquid.
As pipettes are the most precise close to their maximum uptake volume and micro-pipettes are inherently much less precise than pipettes for larger volumes I'd do a two-step dilution.
1. step: 1.000 ml of original solution with 1 ml pipette + 4 times 4.750 ml with 5 ml pipette
2. step: 1.000 ml of solution from step 1 + 4 times 4.750 ml with 5 ml pipette.
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To prepare 20 ml of a 1:400 dilution, you would mix 20 ml of the original solution with 7980 ml (20 ml x 400) of the diluent (such as water or buffer). This will give you a final solution with a concentration that is 1/400th of the original solution.
1 ml solute to 19 mls solvent. This gives a total volume of 20 (20 fold)
20 reams or 10,000 sheets of 20 lb paper would weigh approximately 100 pounds.
The mass of the substance is 20 grams in a 10 ml sample. Therefore, the substance has a density of 2 grams/ml. For a 200 ml sample of the same substance, the mass would be 400 grams (200 ml x 2 grams/ml).
Multiply the two dimensions to get the area. The calculation will give you 80 square feet.
If 1 ml of a 10^-3 dilution is plated, the number of colonies would be 20 colonies (20000 cfu/ml / 10^-3). Whether 20000 cfu/ml is a health hazard would depend on the specific bacteria present and their pathogenicity, as well as individual health conditions. Generally, a high level of bacteria in food products can pose a health risk, especially for vulnerable populations.