Let

be the original amount of the substance. After
t years, the amount left
A is given by:

.
For example, after 1 year, the amount is

.
Thus, after one year, only 0.75=3/4 of the original amount is left.
We want to solve for
t, such that

is one-third of

, so we set the equation:

.
Simplifying by

, we have:

.
This is an exponential equation, so we can solving it by rewriting it as a logarithm:

.
Answer: 3.8