To find the cost of each bag of potatoes, we can set up an equation based on the information provided.
Let \( x \) be the cost of each bag of potatoes.
- She pays a total of $31.69.
- The cost for milk is $4.09.
- She buys 4 bags of potatoes, each costing \( x \).
The total cost can be expressed as the sum of the cost of the milk and the cost of the potatoes. The cost of the potatoes, since she buys 4 bags, would be \( 4x \).
We can write the equation:
\[ 4x + 4.09 = 31.69 \]
Now, we will solve for \( x \):
- Subtract \( 4.09 \) from both sides:
\[ 4x = 31.69 - 4.09 \]
Calculating the right side:
\[ 31.69 - 4.09 = 27.60 \]
So, we have:
\[ 4x = 27.60 \]
- Now divide both sides by 4 to find \( x \):
\[ x = \frac{27.60}{4} \]
Calculating the division:
\[ x = 6.90 \]
Thus, the cost of each bag of potatoes is \( \boxed{6.90} \).