The of the original culture was 4 × 10⁶ CFU/mL.
I presume that you mean 74 CFU (colony forming units).
##”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
“##”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.
