Answer:
The half-life of a substance is the amount of time it takes for half of the initial amount of the substance to decay.
We can use the following formula to calculate the half-life (t1/2) of a substance with a decay rate of r:
t1/2 = (ln 2) / r
where ln 2 is the natural logarithm of 2 (approximately 0.693).
In this case, the decay rate is 2.5% per year, or 0.025 per year. Plugging this into the formula, we get:
t1/2 = (ln 2) / 0.025
t1/2 = 27.73 years (rounded to two decimal places)
Therefore, the half-life of the substance is approximately 27.73 years.