A half-life of two years, means that the original amount will decay to half the original amount in two years.
So, if you have 50 grams of a radioactive isotope with a half-life of 2 years, after 2 years it will have 25 grams. But since we are looking for after 4 years, then you divide it by 4. The correct answer is that it will have C. 12.5 grams of isotope after 4 years.
