The radius of a spherical balloon is tripled. The increase in volume as a percent is.
If the radius is tripled....the volume increases by a cube of this = 27 times
So...if the volume doubles....that's a 100% increase = (2 -1) * 100%
And if it triples....that's a 200% increase = (3 - 1) * 100%
So......an increase of 27 times gives a % increase of (27 - 1) * 100% = 2600%