Electricity about resistance

Question

Two copper wires, A and B, of the same length have the ratio of mass of 4 : 9. Assuming the cross-sectional are uniform, what is the ratio of the resistance of A to that of B ?

The answer is 9 : 4. But why, how to calculate it ?? thx for answering.

Steps are needed.

Answer 1

Let Ma, Mb be the masses of wire A and B respectively
Aa and Ab be their respective cross-sectional areas
and L be their lengths

Thus resistance of wire A, Ra = pL/Aa
where p is the resistivity of copper

resistance of wire B, Rb = pL/Ab

Ra/Rb = [pL/Aa]/[pL/Ab] = Ab/Aa --------------------- (1)

But Ma = density x volume = d.(Aa.L) (where d is the density of copper)
and Mb = d.Ab.L
thus Ma/Mb = Aa/Ab = 4/9

substitute into (1),
Ra/Rb = 9/4