Volume scales as the cube of a linear dimension, so the volume is (10/7.5)^3

times that of the 7.5" cake.

Amount of flour needed scales as the volume, so (10/7.5)^3 times as much

flour is required.

The amount of icing scales as the surface area which scales as the square

of a linear dimension so (10/7.5)^2 times as much iceing is required.

RonL