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 20 days). If it is safe to feed the hay to cows when 7% 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 $LaTeX: \displaystyle k = \frac{\ln 2}{20}$ . This gives the equation $LaTeX: \displaystyle 0.07 = e^{-\frac{\ln(2)}{20}t}$ Taking the natural logarithm of both sides gives $LaTeX: \displaystyle \ln(0.07)= \frac{-t\ln(2)}{20}$ . Solving for $LaTeX: \displaystyle t$ gives $LaTeX: \displaystyle t = -\frac{ 20\ln(0.07) }{ \ln(2) }$ . The farmers had to wait about 76.7 days.