Explanation

For a first-order reaction, the remaining concentration at time t is given by [A]t/[A]0 = (1/2)^(t/t_1/2). At t=3t_1/2, the ratio is (1/2)^3 = 1/8 or 0.125. The question likely asks for the fraction remaining relative to some initial value, but based on the answer A=0.875, it might be asking for the fraction reacted: 1 - 0.125 = 0.875.