What does safety limit mean here?
closed as off-topic by John Rennie, StephenG, user259412, Jon Custer, ZeroTheHero Jun 6 at 11:07
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The safety limit there is the limit above which ionising radiation can cause serious damage to matter, particularly living tissues. Crossing this limit is dangerous, making it necessary to control our exposure to such radiation.
Now, in case you are also looking for some explanation for the answer:
For that problem, you don't really need to know much about what exactly the safety limit is, just that it exists and that after a half-life a source will emit, on average, half the radiation it was emitting at the start.
Therefore, after one half-life, the radiation will still be 16 times above the safety limit. After 5 half-lives, or 15 hours, the radiation will have halved 5 times, going from 32 $\rightarrow$ 16 $\rightarrow$ 8 $\rightarrow$ 4 $\rightarrow$ 2 $\rightarrow$ 1, which means the radiation is right around its safety limit.