Question

A scientist has a 120 mg sample of an unstable isotope. The amount of the substance remaining in the sample decreases at a rate of 16% each day. At the end of t days, there is less than 30 mg of the substance remaining. Which inequality represents this situation, and after how many days will the sample size remaining be less than 30 mg?

Answer 1

Explanation:

Got it correct on Edmentum

Answer 2

after each day there is 1 - 0.16 = 0.84 of the substance left
The inequality is 120*0.84^t < 30

120*0.84^t < 30
0.84^t < 30/120 < 0.25
t * ln 0.84 < ln 0.25
t < ln 0.25 / ln 0.84
t < 7.95
Answer = 8 days