Ahalf life of cobalt is 5.27years. approximately how much a 399 g sample will remain after 20 years?

= 28.745 g

Explanation:

Half life is the time taken by a radioactive element to decay by half the original amount.

Therefore;

New mass = Original mass × (1/2)^n

Where n is the number of half lives, in this case, n = 20/5.27 = 3.795

Therefore;

New mass = 399 g × (1/2)^3.795

                = 28.745 g

a is the correct answer