After the release of radioactive material into the atmosphere from a nuclear power plant the hay in that country was contaminated by a radioactive isotope (half-life 25 weeks). If it is safe to feed the hay to cows when 18% of the radioactive isotope remains, how long did the farmers need to wait to use this hay? Round to the nearest tenth.

The decay constant is $LaTeX: \displaystyle k = \frac{\ln 2}{25}$ . This gives the equation $LaTeX: \displaystyle 0.18 = e^{-\frac{\ln(2)}{25}t}$ Taking the natural logarithm of both sides gives $LaTeX: \displaystyle \ln(0.18)= \frac{-t\ln(2)}{25}$ . Solving for $LaTeX: \displaystyle t$ gives $LaTeX: \displaystyle t = -\frac{ 25\ln(0.18) }{ \ln(2) }$ . The farmers had to wait about 61.8 weeks.