If N0 is the original mass of the substance of half-life period t1/2 = 5 years, then the amount of substance left after 15 years is
Amount left after decay is given by N
N = N0(1/2)n
n = number of half lives = t/t1/2= 15/5 = 3
Amount left = N = N0(1/2)3
The correct option is B.