# need a math check - (Feb/14/2011 )

Please check my math

I have 1ul of 10ng/ul solution, I add 19ul water to it so it is now 0.5ng/ul at 20ul total volume (where 1/20*10 = 0.5)

Next I take 2ul of the 0.5ng/ul and do a 1:5 dilution, so it is now 0.1ng/ul (where 1/5*0.5=0.1) at 10ul total volume.

But now I want more than 0.1ng/ul, so I add in 5ul more of the 0.5ng/ul. Now it's 15ul total volume, not 10ul anymore. What is the new concentration after this 5ul addition? I have that since I added 7ul total of 0.1ng/ul, it is (7/15)*0.5=0.2ng/ul. Is this correct and is there another way to do it?

Hola, Only at the end you have, to me, the confusion 5ul of 0.1ng/ul in 15 ul are 0.5ng/15ul<> 500pg/15ul<>33.3pg/ul. If they are 7ul/15 means 0.7ng/15ul<> 700pg/15ul or 46.6 pg/ul. Buen día

i think: if you have 0.1 ng/ul at 10 ul--> is 1 ng. You add 5 ul of 0.5 ng/ul, so you add 2.5 ng. Your total is 3.5 ng at 15 ul, so 3.5/15= 0.233333 ng/ul

fysio lab on Tue Feb 15 09:35:45 2011 said:

i think: if you have 0.1 ng/ul at 10 ul--> is 1 ng. You add 5 ul of 0.5 ng/ul, so you add 2.5 ng. Your total is 3.5 ng at 15 ul, so 3.5/15= 0.233333 ng/ul

hi, ok so if i understand you correct, you added it in terms of how many ng is being put into the 15ul in the end.

so the first time, i added 2ul of 0.5ng/ul = 1ng into 10ul volume

then second time i added 5ul of 0.5ng/ul = 2.5ng

2.5ng+1ng = 3ng

3ng/15ul volume = 0.2333

but also my math gives the same (7/15)*0.5ng/ul = 0.2333ng/ul which i simplified to 0.2ng/ul

would like to know what others say. we get the same answer, so do both our approaches work or just an odd coincidence? thanks

double post deleted

molbio1234 on Tue Feb 15 10:47:33 2011 said:

I´m terribly sorry, you two are correct, I´M wrong by a bad interpretation. Sometimes is better stay in silence. I hope you two can excuse me . Thanks

your way is also correct, different people--> different way of calculations