Let the radius of cone be r and height be h.
Therefor initial volume of cone(V) = 1/3
After increasing the height and radius of cone by 20%. We get the value of:-
Radius = (100+20)% of r= 120% of r = 120/100*r = (12/10 *r)
And height = (100+20)% of h= 120% of h = 120/100*h = (12/10 *h)
Now the new Volume (Vn) will be = 1/3 *
Now the percentage increased = (Vn - V )/V * 100 = 72.8%