Energy levels in transition metals

[DiligentDoc]

1. The problem statement, all variables and given/known data
Which is lower in energy in transition metal ions, the 3d orbitals or 4s orbitals?


The answer is 3d orbitals but I would like to know why 3d orbitals in transition metal ions are lower in energy than in 4s orbitals (despite the 4s orbitals coming before the 3d orbitals on the periodic table).

Thanks