1. The problem statement, all variables and given/known data The half-life of a radioactive substance is 194 days. How many days will it take for 80% of the substance to decay? 2. Relevant equations 3. The attempt at a solution I had to make 1/2n=1/5. I did this by finding log25=n. After this, I needed to only multiply by the number of days it took the half-life to occur, which was 194, and got 451 (approx.) as a reasonable answer.