I have this question which I believe I solved correctly, but I was wondering if someone could look it over to make sure?

Question

The question is...

Assay for Vitamin A (a fat soluble vitamin) required an extraction step into ether. In this assay, 2.00 mL of serum are extracted into 5.00 mL of ether. The extraction step is 100% efficient. Ether and serum are immiscible. The ether layer is separated and taken to dryness. The remaining residue is the resuspended in 10 mL of methanol. The concentration in this final solution is 10 ng/mL. What is the concentration of the Vitamin A in serum using units of Moles/liter? (Vitamin A M.W. = 450)

My solution:

10ng Vitamin A/10mL * 1g/1x10^9ng * 1000mL/1L * 1mol/450g Vitamin A

= 10,000 mol/4.5x10^12L

= 2.22x10^-9 mol/L

Answer 1

I may have missed something but I believe this needs rethinking.
As I read the problem, the final solution concn is 10 ng/mL (and not 10 ng/10 ml). I assume this is 10 ng/mL in the 10 mL methanol solution.
Then 10 ng/mL x 10 mL = 100 ng in the final 10 mL methanol soln. That is 100 ng in the original 2 mL solution or 50 ng/mL in the original sample. Convert that to M.

Answer 2

You're right. My mistake!

Thank you...