To find the total cost \( c \) that Vicky paid for the oranges, we can set up the equation based on the given information.
Vicky bought \( \frac{3}{12} \) pounds of oranges, which can be simplified to \( \frac{1}{4} \) pound. Each pound costs $3.90.
The total cost \( c \) can be calculated using the following equation:
\[ c = \text{(weight in pounds)} \times \text{(cost per pound)} \]
Substituting the values we have:
\[ c = \frac{3}{12} \times 3.90 \]
or in its simplified form:
\[ c = \frac{1}{4} \times 3.90 \]
This equation represents the total cost Vicky paid for the oranges.