Question

On Monday you helped me to understand and answer the one question, but I am not too sure if they want all the calculations that you answered... regretably, I am still a bit lost with these!

Below is the question as it is printed:

The isotope caesium-137, which has a half-life of 30 years, is a product of nuclear 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 your answer. (A few sentences)

.......... the reason why I think I do not need the calculations is because it asks for a FEW SENTENCES, am I right in my thinking? If so, can you pls help me with what I should write?

If I remember the problem right, I think I worked it incorrectly ( that is I don't remember how I worked it but I seem to remember an answer of 40+ years). I don't know what went wrong, assuming I remember correctly. The correct answer is 120 years.
It will take 30 years for 1/2 of Cs-137 to decay (leaving 1/2), 60 years for 1/2 of that to decay (leaving 1/4), 90 years for 1/2 of the 1/4 to decay (leaving 1/8), and 120 years for 1/2 of the 1/8 to decay (leaving 1/16).