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

1. N0/16
2. N0/8
3. N0/4
4. N0/2

Amount left after decay is given by N

N = N0(1/2)n

n = number of half lives = t/t1/2= 15/5 = 3

Therefore,

Amount left = N = N0(1/2)3

= N0/8

The correct option is B.