C. $34.50. A car gets 30 miles per gallon, if the gasoline cost $1.15 per gallon, a 900 miles trip will cost $34.50
This exercise can be solved by simply arithmetic.
Divide the total miles by the miles per gallon:
900/30 = 30
Then, multiply this value by the cost per gallon:
30 x $1.15 = $34.50