Official Solution:What was Silvia's income in December? (1) In December, Silvia's income was $50 more than her average (arithmetic mean) monthly income for the year.
The above gives the following equation: \(December \ income = Average \ income + $50\). Not sufficient.
(2) In December, Silvia's income was twice her average (arithmetic mean) monthly income for the year.
The above gives the following equation: \(December \ income=2*Average \ income\). Not sufficient.
(1)+(2) We have two distinct linear equations with two variables, \(December \ income = Average \ income + $50\) and \(December \ income=2*Average \ income\), hence we can solve it. Sufficient.
Answer: C