# Math Help - Radioactive Decay

If Forty percent of a radioactive substance decays in five years. By what percent does the substance decay each year?

2. Originally Posted by Royster123
If Forty percent of a radioactive substance decays in five years. By what percent does the substance decay each year?
Exponential Decay: $N(t) = N_0e^{-\lambda t}$

$0.6 = 1e^{-40\lambda}$

$\lambda = -\frac{1}{40}ln(0.6)$

$
N(t) = 1e^{-\left(-\frac{1}{40}ln(0.6)\right) \times 1)}$