lets calculate how many miles did he rode his bike, that would be 18 times 9.8 miles:
total miles = 18*9.8
total miles = 176.4
that is the total miles he rode his bike, now lets find out how many gallons that represents. If 15 miles equals a gallon, then how many gallons are 176.4 miles, we can solve that with a rule of three:
15 mi -----> 1 gal
176.4 mi --> x
x = (176.4)(1)/15
x = 11.76 gal
therefore he saved 11.76 gallons of gasoline by riding his bike 176.4 miles, and that represented an amount of dollars of:
11.76 * 3.75 = 44.1
that is because each gallon costs $3.75