Answer:
$52.50
Explanation:
Set up the ratio:

Simplify in order to find the cost per gallon.
The cost per gallon is $3.50 because 7 divided by 7 is one and 24.50 divided by seven is 3.5.
Now you have this left:

Multiply the denominator by 15 since Robert needs 15 gallons.
Then, multiply the top by 15 because whatever you do to the denominator, you have to do to the top.
Now you have this left:
=

Therefore, Robert needs to pay $52.50 in order to recieve 15 gallons of gasoline.