# Question e4ec1

Oct 4, 2014

The density of the original culture was 4 × 10⁶ CFU/mL.

I presume that you mean 74 CFU (colony forming units).

$\text{74 CFU"/"0.2 mL sample" × "10 000 mL sample"/"1 mL original}$

= 4 × 10^6"CFU/mL of original solution"

Note: The answer can have only 1 significant figure, because that is all you gave for the size of your aliquot. If you need more precision, you will have to recalculate.

The formula I find in my searches is

"$\text{CFU" × "dilution factor" × 1/"aliquot" = "CFU/mL}$"

Then 74 × 10 000 × 1/0.2 =4 × 10^6 " CFU/mL"#

It gives the same numerical answer, but it requires memorization.