A 200-gram sample of a radioactive nuclide is left to decay for 60 hours. At the end of 60 hours, only 25 grams of the sample is left. What is the half-life of the nuclide sample?

60 hours

30 hours

40 hours

20 hours - - - > I think 20?

Half life is the time taken by a radioactive isotope to decay by half its original mass.

The original mass is 200 g

Time taken is 60 hours

Final mass is 25 g

Therefore;

Final mass = Original mass * (1/2) ^n; where n is the number of half lives.

25 = 200 (1/2) ^n

1/8 = (1/2) ^n

n = 3

Three half lives = 60 hours

1 half lives = 20 hours

Therefore; the half life of the radioactive nucleus is 20 hours