divide 6 by 2 and multiply by 100 (because that's the scale). You should get 300. Then, divide 3.2 by 2 and multiply by 100. You should get 160. Then, because the area would be 300*160, your answer is 48000
This is pretty simple. Since there are 19.8 gallons and every gallon is 3.65$, you multiply 19.8 by 3.65 to get $72.27. It will cost 72.27$ to fill up the tank.
