hi, im a newbie at this website, and a first year biochemistry student, and i am having a difficulty in regards to the following question

"a reaction is designed to be carried out ina 5 ml total volume. It requires a a final NAD concentration of 6mM. If the protocol allows 90 ul for the NAD addition, what concentration would you make up the stock solution?"

if someone could guide me in to answering this question, that would be great. Im not a genius so please don't make the answer sound complicated.

Cheers
Dave

i don't recognise the unit "ul". what is that?

u often stands for greek ? (mi, 10^-6). There is no ? on keyboard and most peaople doesn't know how to enter it. And even if they know, it is often diplayed as some bush.