One of the main contaminants of a nuclear accident, such as that at chernobyl, is strontium-90, which decays exponentially at an annual rate of approximately 2.8% per year.

From the statement you can find the function that represents the percent of strontium-90 that remains after t years of the nuclear accident.

Call P the percent:

---> P = e ^ (- 0.025t)

Now you can also estimate the half-life of the isotope strontium-90:

Call Pi the initial percent and P2 the half percent:

P2 = 0.5 * P initial =>

P2 = 0.5 * Pinitial = Pinitial * e ^ (-0.025 t)

=> 0.5 = e ^ (-0.025t)

=> ln 0.5 = -0.025t

=> t = - ln (0.5) / 0.025

=> t = 27.7 years < half-time