The lenghts of diameters of the two hollow spheres are 21 cm and 17.5 cm.
`therefore` their radii are `(21)/(2) "cm and" (17.5)/(2)` cm
So, the ratio of the curved surface areas of the two hollow sphere
`=4pixx((21)/(2)):4pixx((17.5)/(2))^(2)= (21^(2))/(4):(17.5)^(2)/(4)=21xx21:17.5xx17.5=(21xx21xx100)/(175xx175)=(36)/(25)=36:25`