Well, don't let it bring you down, quite a few people would be stymied by this question, similar questions are quite common on this board.. I told you, if they plopped this on my desk at work, I'd be overwhelmed too -- for a little while. Usually, once the calculation has been worked out, it's been worked into the integration program of the HPLC software, and no one has to look at it again. It's nice, from the employer's point of view, to see that you're versatile enough to handle the problem, in case something goes wrong.
You'll notice, that I didn't work out the whole thing for you -- if I were to, and double and triple check it to be sure it worked right -- I'd expect to be paid, and of course, so do you, eventually. Give them this freebie, and let them know, you'll be looking forward to more problems like this, so you get the job. Once you've done that, you can brush up on these skills, and be in high demand for jobs with these sorts of tasks.
And if you get it wrong -- own the error, make it yours, make it work for you. Admit you're not perfect, show them you want to learn, and let them know they can't spook you with a big block of text.
Oh, and uh, when you get a chance, can you point me to the online test? Don't worry, I'm not a Canadian citizen, so I'm not gunning for your job, I just want to take the test myself, and see if I'm good enough. It could come in handy with my own job search.