Remark
This is an important question to understand. This and the number of miles you get per gallon. You can develop a proportion or you can make up your own equation. I prefer a proportion.
Formula
1 gallon = 3.65 dollars.
19.8 gallons = x
1/19.8 = 3.65 / x Cross multiply
1*x = 19.8 * 3.65
x = 72.27 dollars. The fly in the ointment is what does the question mean by "About"? Does the question want us to approximate? Or round? If it does then try 72 dollars.