The half-life of Cesium-137 is 30 years. Suppose we have a 140-mg sample.
i. What is the function that describes how much is left after t years?
ii. How much of the sample remains (mg) after 8 years?
iii.After how many years will only 10 mg remain?

I'll give you the answer to part 1, and then from that you can get the other 2. If you start with 'N' grams of material which has a half-life of 'H' years, then after 't' years the remaining is:


x = N(\frac 1 2 )^{t/H}


For part 3 you will have to use logarithm operations to solve for 't' given 'x'.