Question

Suppose you have 50 grams of a radioactive isotope with a half-life of 2 years. How much of the isotope will you have after 4 years? 50 25 12.5 6.25 grams

Answer 1

The answer is 12.5 grams because after 2 years, 50 g goes to 25 g, then after another 2 years, it goes to 12.5 g

Answer 2

50 / 2 = 25
25 / 2 = 12.5 

In four years three will be 12.5 g of the radioactive isotope left 

Hope this helps :)