In case of Cr3+ ion , the valence shell electronic configuration is same whether the ligand is strong field or weak field.
The spin only magnetic moment , μeff=n(n+2)BM
The valence shell electronic configuration of Cr3+ is 3d3, hence, it has three unpaired electrons.
μeff of [Cr(CN)6]−3=15 B.M.
μeff of [Cr(H2O)]6+3=15 B.M.
Ratio =1