Let the cost of one soda be \( s \).
According to the problem, the total cost of 5 sodas can be expressed as:
\[ 5s \]
We are also told that this total cost is equal to the cost of 3 sodas plus $7. The cost of 3 sodas can be expressed as:
\[ 3s + 7 \]
Setting the two expressions for the total cost equal to each other, we have:
\[ 5s = 3s + 7 \]
To solve for \( s \), subtract \( 3s \) from both sides:
\[ 5s - 3s = 7 \]
This simplifies to:
\[ 2s = 7 \]
Now, divide both sides by 2:
\[ s = \frac{7}{2} = 3.5 \]
Thus, the cost of one soda is
\[ \boxed{3.50} \]