Question

Please explain how to get the answer.

Answer 1

Alrighty..

Volume of sphere = 4/3 x (pi)x r^3 .where r is the radius of a circle ..in this case the radius for the smaller circle. and the Volume of the larger one would be 4/3 x (pi)x R^3 .
so when they say the volume if the larger one is Twice the smaller one they're bascially telling you that (4/3x(pi)xR^3 = 2 x (4/3x(pi)xr^3) 

When you multiply the formula by two you'd get 4/3x(pi)xR^3= 8/3 x (pi)x r^3 

Now solve accordingly by taking 4/3 to the other side.. you'll get sth like this 

(pi)R^3 = 2(pi)r^3  
Cancel the pi out 
 
now you have R^3 = 2r^3  divide the equation now by r^3 
 R^3/r^3 =2
you can also write this as (R/r)^3 = 2 
take the power cube to the other side to get cube root of 2!!!
therefore R/r= cube root of 2 

Hope this helped! :)

Answer 2

2r=R

Because 2 of "r" make 1 R