1. The problem statement, all variables and given/known data If 20% of a radioactive substance disappears in 70 days, what is its half-life? 2. Relevant equations y = C*e^(k*t) where t is time in days k is the constant of proportionality? y is the current amount of substance 3. The attempt at a solution 20% disappears, so 100% - 20% = 80% remaining = 0.80 after 70 days .8*C = C*e^(70*k) Solve for k k = -3.188 * 10^-3 Plug k value back into equation to find t at the substance's half-life (.5) .5*C = C*e^((-3.188 * 10^-3)*t) Solve for t t = 217.44 217.44 days is the substance's half-life. Is that correct? Thank you.