I would like to know which one is the right formula to calculate the decay if the book is asking to find the remaining radioactive salt after 75 years, knowing that the decay rate is 2% and we had 100 kg at the beginning. The book's result is 22kg after 75 years, so which one of these 2 formulas should be the right one?
or
Ps. with both of them, I get pretty close the book's answer
Thanks