Three concentric metallic sphere shells of radii R,2R,3R are given charges Q1,Q2,Q3 respectively. It is found that the surface charge densities on the outer surfaces of the shells are equal. What is the ratio of the charges given to the shells Q1,Q2,Q3?

Answer 1

sigma=Q/area

but areas are proportioal to radius squared.

so if the sigmas are equal, then

Q1/r1^2=Q2/r2^2=Q2/r3^2

I assume you can take it from there.

Answer 2

1:4:9