Question:If the ratio of volumes of two sphere are in the ratio of `4 : 25,  then the ratio of their volumes is : 

A `1 : 2` 
B `1 : 4` 
C `1 : 8` 
D `1 : 16` 

+ Answer
B 

+ Explanation
Let their radii be R and r. Then,

`(4/3 π R^3)/(4/3 π r^3)`

`1/8 ⇒ (R/r)^3`

`= 1/8 = (1/2)^3.`

So, R/r = 1/2.`

Ratio of surface areas 

`= (4 πR^2)/(4πr^2)`

`= (R/r)^2`

`= 1/4.`

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Irin Moni

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