The half-life of tritium, or hydrogen-3, is 12.32 years. after about 24.6 years, how much of a sample of tritium will remain unchanged?
a. 1/8
b. 1/4
c. 1/3
d. 1/2

The amount of the substance left after sometime, t, is given by the equation,
                                     At = (Ai) x e^-kt
where Ai is the initial amount and k is constant. From the given half-life,
                            At / Ai = 0.5 = e^-k(12.32)      ; k = 0.5626
Then, for the next set,
                             At/Ai = e^(-0.5626)x24.6 = 
Thus, the answer is letter B.