You don't have a picture we can help with
The equation would be .15x + .30(5-x) = .2(5)
Solving I got 3 1/3 gal 15% gloss, 1 2/3 gal 20% gloss
The radius is the line from the center of circle to any point in the circumference, and diameter is the line that goes through the center of the circle from one point of the circumference to another. The relationship between them I will say that the radius is half of the measure of the diameter.
Add the numerators and keep the denominator: