Half-lifes are pretty fun, atleast this one since its a straight out problem, here let me show you.
So we are starting with 30mg/L one half life will reduce it down half of 30 thus...
30/2 = 15 So, thats 16 hours now again
15/2 = 7.5 do it again
7.5/2 = 3.75 again,
3.75/2 = 1.875 again,
1.875/2 = 0.9375 again...
0.9375/2 = 0.46875 again,
0.46875/2 = 0.2343 again,
0.2343/2 = .1172 again
0.1172/2 = 0.0585 which its really close to the mark but Im going to assume that its asking for 0.0500 mg/L so again.
0.0585/2 = 0.0292 and boom we have our mark, now it can't be detected. So now count all the times we had to half life.
The total times is 10 times, which we just do 16 hours x 10 = 160 hours until you can't detect.
That sure is a long time, and that is always an easy way to remember, if you hear half-life just keep knocking the numbers down. I hope this helps a little bit :+)