If the radius of a sphere is tripled,

Volume of a sphere V= $ \frac{4}{3}\pi r^3 $
Original radius = r
New radius = 3r
Original volume = $ \frac{4}{3}\pi r^3 $
New volume = $ \frac{4}{3}\pi (3r)^3 $
=$ \frac{4}{3}\pi \cdot 27r^3 $
=$ 27\left(\frac{4}{3}\pi r^3\right) $
Ratio of volumes = $ r^3 : (3r)^3 $
=1 : 27
Answer = 1:27