Over a certain time period, did the number of shares of stock in Ruth's portfolio increase?Let the number of stocks be \(s\) and the number of bonds be \(b\). The question is: Did \(s\) increase?
(1) Over the time period, the ratio of number of shares of stock to the total number of shares of stocks and bonds in Ruth's portfolio increased.
This implies that \(\frac{s}{s+b}\) increased. This information is not sufficient, as it is possible that \(s\) increased while \(b\) remained the same, or \(s\) remained the same while \(b\) decreased.
(2) Over the time period, the total number of shares of stocks and bonds in Ruth's portfolio increased.
This implies that \(s+b\) increased. This information is clearly insufficient on its own.
(1)+(2): \(\frac{s}{s+b}\) increased, and \(s+b\) (the denominator) also increased, so it must be true that \(s\) (the numerator) increased as well. Therefore, this is sufficient.
Answer: C.
Hope it's clear.