Need helping solving a Dilution Problem

[Sohzi]

Plaque assay is performed beggining with a 10 mL phage solution. The solution is serially diluted 4 times by taking 1 mL and adding it to 9 mL. 0.1 mL of the final dilution is plated & yields 21 plaques. What is the initial density of the starting phage solution?

 

So here's what I know. Density= (# of colonies formed/ mL plated) x Dilutionbefore plating

 

# of colonies formed is 21.

 

Now I'm assuming the mL plated is 0.1 mL?

 

And as for the the dilution I am stuck. Is there a way I can figure out how to calculate that?

[CharonY]

You take 1 ml and fill it up to 10 ml. What amount of dilution is that?

[Sohzi]

So if we do that four times it becomes 10000 more dilute. Is that my dilution number I'm looking for?

[CharonY]

Yupps.

[Sohzi]

Wow awesome.

 

 

So...

 

(21 plaques/ 0.1 mL plated) x 10^5

= 2.1x10^7 PFU/mL

 

So I did it perfect? 100%?

[T4phage]

yeah, that's what I got when I did it on the assignment