What would the final concentration (cells/ml) be if you made a 1:3 dilution from a culture of 1x10e9 cells/ml?
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Sometimes it isn't clear what people mean when they express dilutions this way--1:3--
If by 1:3 you mean 1 part cells to 3 parts media, i.e. 1 part in a total of 4 (which is what I think of when I say 1:3), this would be a 1/4 dilution, it would be 1/4 = 0.25x10^9. But sometimes people say 1:3 when they mean a 3-fold dilution, which is really a 1:2. In that case your conc would be 1/3 of the original, i.e. 0.33 x 10^9 cells/ml. You figure out which one you mean, and pick the answer from the two options I gave you.
If by 1:3 you mean 1 part cells to 3 parts media, i.e. 1 part in a total of 4 (which is what I think of when I say 1:3), this would be a 1/4 dilution, it would be 1/4 = 0.25x10^9. But sometimes people say 1:3 when they mean a 3-fold dilution, which is really a 1:2. In that case your conc would be 1/3 of the original, i.e. 0.33 x 10^9 cells/ml. You figure out which one you mean, and pick the answer from the two options I gave you.