The radius of a copper-sphere is twice the radius of an iron-sphere. The numerical value of the whole surface area of the copper-sphere is equal to the numerical value of the volume of the iron-sphere. Then the radius of the copper-sphere will be

Options

A. 6 unit
B. 12 unit
C. 18 unit
D. 24 unit

Explanation

Let the radius of the iron-sphere be r units.
`therefore` the radius of the copper-sphere is 2r units.
`therefore` the whole surface area of the copper-sphere `4pixx(2r)^(2) "sq-units" =16 pir^(2)` sq-units
Also, the volume of the iron sphere`=(4)/(3)pir^(3)` cubic-units
As per question, `(4)/(3)pir^(3) =16pir^(2) rArr r =12`
`therefore` the radius of the copper-sphere=2xx12 units 24 units.
`therefore` (d) is correct.