dilution question - is this correct? (Jan/06/2011 )

Can someone just check my answer:

I have 7ul volume of 7ng/ul stock solution = 49ng total amount
I want 10ul volume though, so I add 3ul of water into the 7ul. I need the new total amount.

So my reasoning is if I had 7ul of water instead of 3ul, I have diluted the stock 1:1 so the new total amount is 24.5ng (49*0.5). If I just add 3ul of water, I have diluted the stock down 0.7x, so the new amount is 34.3ng (0.7*49). Is this correct? Is there another way to do this?

if you add 3 µl of water, you still have 49 ng of your compound, this doesn't change no matter how much water you add

the only thing that changes is the concentration, in the beginning you had 7 ng/µl, now you have 49 ng in 10 µl ==> 4,9 ng/µl