We know that:

is an equation of a circle.
When we substitute x and y (from the pairs we have), we'll get a system of equations:

and all we have to do is solve it for a, b and r.
There will be:

From equations (II) and (III) we have:

and from (I) and (II):

Now we can easly calculate a and b:

Finally we calculate

:

And the equation of the circle is:
18.5 - 6 = 12.5 gallons empty
12.5 / 18.5 = 0.676
0.676 x 100 = 67.6%
Answer: 67.6%
Answer:

Step-by-step explanation:

<em>hope</em><em> </em><em>this</em><em> </em><em>helps</em><em> </em><em>you</em>
<em>let</em><em> </em><em>me</em><em> </em><em>know</em><em> </em><em>if</em><em> </em><em>you</em><em> </em><em>have</em><em> </em><em>another</em><em> </em><em>questions</em><em>:-)</em>