If N_{0} is the original mass of the substance of half-life period t_{1/2} = 5 years, then the amount of substance left after 15 years is

- N
_{0}/16 - N
_{0}/8 - N
_{0}/4 - N
_{0}/2

**Answer**

Amount left after decay is given by N

N = N_{0}(1/2)^{n}

n = number of half lives = t/t_{1/2}= 15/5 = 3

Therefore,

Amount left = N = N_{0}(1/2)^{3}

= N_{0}/8

**The correct option is B.**