It's difficult to answer because we don't know the cutoff point for whatever your test is going to be. But find the half life (wanna say it's like 5 hours or so, and keep dividing by 2 until you get to a number low enough that's under your cutoff point, along with some basic conversions in the metric system.
I don't think these questions are allowed so I'm not going to go into too much detail
Sorry. But good luck. I've been in that situation and I know how stressful it is.