Suppose a laboratory has a 61 g sample of polonium-210. the half-life of polonium-210 is about 138 days.
a. how many half-lives of polonium-210 occur in 828 days?
b. how much polonium is in the sample 828 days later?

To answer for the first question, divide 828 days with the half-life, 138 days. There are 6 half-lives of polonium-210 in 828 days. 

For the second question, raise 0.5 to number of half-lives which was calculated in a, 6. Then, multiply with the initial amount, 61 g. Thus, after 828 days there are only about 0.9531 g of polonium-210 left. 

The answer is 109 times per second