Using an exponential function, it is found that it takes 18.6 days for 80% of the material to decay.
The amount of a substance after t days is modeled by:

In which:
- k is the decay rate, as a decimal.
- A(0) is the initial amount.
The half-life is of 8 days, hence
, and this is used to find k.







Hence:

The time it takes for 80% of the material to decay it t for which
, hence:






It takes 18.6 days for 80% of the material to decay.
To learn more about exponential functions, you can take a look at brainly.com/question/24906920
First you need to solve for x by subtracting 9 from both sides (-3x less than/equal to 6), then dividing the inequality by -3, which would also flip the sign, so to speak. now you would have x is greater than/ equal to -2. now you would draw a circle above -2, fill it in since it is equal to, and draw the arrow going toward 0 (I think)