The percentage of the original sample that is left after 115 days is; 23.4733%
Formula for radioactive decay is;
A = A₀ * (1/2)^(t/t_¹/₂)
where;
A = quantity of the substance remaining
A₀ = initial quantity of the substance
t = time elapsed
t_¹/₂ = half life of the substance
Thus;
A/A₀ = (1/2)^(115/55)
A/A₀ = 0.234733 or 23.4733%
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