To find the total cost \( c \) that Vicky paid for the oranges, we can use the following equation:
\[ c = \text{(cost per pound)} \times \text{(number of pounds)} \]
In this case, Vicky bought 312 pounds of oranges, and each pound costs $3.90. Thus, we can substitute these values into the equation:
\[ c = 3.90 \times 312 \]
Therefore, the equation representing the total cost \( c \) that Vicky spent on the oranges is:
\[ c = 3.90 \times 312 \]
You can calculate \( c \) by multiplying the two values to find out the total amount she spent on the oranges.