The number of new flu in a major city decreased from 3,450 to 2,811 within one week. What is the percentage decrease?

First, you should need to know the difference between 'simple' and compound interest.  the basic difference between the two is the that in simple interest the interest is applied to the original amount (even after multiple months). however, in compound interest, the interest is applied on the amount + interest from the previous month.  so in your problem (which is a compound interest problem), after 1 month the amount payable by miriam will be:   650 + (650 * 0.017) = 661.05  next month the interest will be the on 661.05. so after 2nd month  661.05 + (661.05* 0.017) = 672.29  and so c is the best answer