Hello, av8or91!

The half-life function is: .A radioactive substance has a half life of 18 years. Decay rate remains constant.

How long will it be before 95 percent of the sample has decayed?

. . where is the intial amount and .

Our function is: .

If 95% of the sample has decayed, then: .

We have: .

Take logs: .

. .