Question

The isotope caesium-137, which has a half life of 30 years, is a product of neuclear power plants. How long will it take for the amount of this isotope in a sample of caesium to decay to one sixteenth of its original amount. Explain answer

30 years 1/2
60 years 1/4
90 years 1/8
120 years 1/16????

IS this right

looks right to me.