a) $$\sqrt 2 $$ : $$\sqrt 3 $$
b) 2 : 3
c) 4 : 9
d) 16 : 81
Answer: c
Explanation: Let their radii be R and r
$$\eqalign{
& \frac{{\frac{2}{3}\pi {R^3}}}{{\frac{2}{3}\pi {r^3}}} = \frac{{6.4}}{{21.6}} \cr
& {\left( {\frac{R}{r}} \right)^3} = \frac{8}{{27}} \cr
& {\left( {\frac{R}{r}} \right)^3} = {\left( {\frac{2}{3}} \right)^3} \cr
& \frac{R}{r} = \frac{2}{3} \cr} $$
Ratio of curved surface area :
$$ = \frac{{2\pi {R^2}}}{{2\pi {r^2}}} = {\left( {\frac{R}{r}} \right)^2} = \frac{4}{9}\,or\,4:9$$
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