To set up a proportion using fractional notation to find out how much 5 gallons of gas will cost (where \( x \) represents the unknown cost), we can use the information given about the cost of gas.
The proportion can be written as:
\[ \frac{15}{45} = \frac{x}{5} \]
So the correct response is:
Start Fraction 15 over 45 End Fraction equals Start Fraction x over 5 End Fraction