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Using a proportional relationship, it is found that the area is of 637 square inches when the radius is of 7 inches.
A proportional relationship is a function in which the output variable is given by the input variable multiplied by a constant of proportionality, that is:
[tex]y = kx[/tex]
In which k is the constant of proportionality.
In this problem, the surface area of a balloon varies directly as the square of mits radius, hence:
[tex]S = kr^2[/tex]
The area is 325 in² when the radius is 5 in, hence we use it to find r.
[tex]S = kr^2[/tex]
[tex]325 = 25k[/tex]
[tex]k = 13[/tex]
Hence:
[tex]S = 13r^2[/tex]
When the radius is of r = 7 in, the area in square inches is given by:
[tex]S = 13r^2 = 13 \times 49 = 637[/tex]
More can be learned about proportional relationships at https://brainly.com/question/10424180