Question 622288
well you've stated the equation to solve for the volume, 
In this case, you know the volume, so plug it into the equation. 

{{{20=(4/3)(pi)r^3}}}
multiply both sides by 3 and divide both sides by 4
{{{15=(pi)r^3}}}
now divide by pi
{{{15/pi=r^3]]]
now raise everything to the {{{1/3}}} power
{{{(15/pi)^(1/3)=((r^3)^(1/3))}}}
{{{r=(15/pi)^(1/3)}}}