(16 points) another substance has a half-life of 3 hours. what fraction of the atoms will have decayed after 6 hours? plz show work

Halflife is the time taken by a radioactive substance by half its original mass. In this case the half life of the substance is 3 hours. 
Therefore; New mass = Original mass × (1/2)^n where n is the number of half lives. In this case, the half life is 3 hours and therefore; the number of half lives in 6 hours will be two.
 = if the original mass is 100%
new mass = 100 × (1/2)^2
                 = 100 × 1/4 = 25%
The remaining mass will be 1/4 of the original mass, meaning  a fraction of 3/4 of the  initial mass will have decayed.
Thus; the answer is 3/4