A radioactive sample's mass, in grams, after t hours is modeled by M(t) = 1000(0.85)^t. According to this model, by what percent does the…

In $M(t) = 1000(0.85)^t$, the base 0.85 is the factor by which the mass is multiplied each hour. Since $0.85 = 1 - 0.15$, the mass keeps 85% of its previous value and loses 15%. The answer is C. Choice A (85%) is the percent that remains, not the percent lost. Choice B (0.85%) misreads the decimal 0.85 as a percent. Choice D (8.5%) shifts the decimal point incorrectly. For decay, subtract the base from 1, then convert to a percent: $1 - 0.85 = 0.15 = 15\%$.