Half life is the time taken by a radioactive isotope to decay by half its original mass. In this case, the halflife of the radioactive isotope is 5000 years. Initially the mass is 100 %; thus the mass that will be left will be given by; New mass = Original mass × (1/2)^n where n is the number of half lives; n = 10000/5000 = 2 New mass = 100% ×(1/2)^2 = 100 % × 1/4 = 25% Therefore; the mass left after 10000 years is 25% or 1/4 of the original mass.
