Explanation:

Let the volume of spherical balloon initially = V
New volume after increase = V + 700*V/100 = 8V
Since we know that volume of sphere is directly proportional to the radius of sphere

Final radius = 2* initial radius
Since surface area of sphere is directly proportional to the square of the radius of sphere,()

Final surface area = 4*initial surface area
% change = (final area - initial area) / initial area ×100 =( 4-1)/1x100 = 300%