One half life if how long it takes for half of a sample of a radioactive element to decay. If the starting sample is 40 grams, and the half life is 100 years, and it is left for 200 years, there has been 2 half lives, since 200÷x=100 and x =2. Because there has been 2 half lives, you divide the starting amount by 2, twice. And 40÷2÷2=20÷2=10. Answer choice A