What does it mean that an electron quantum numbers are n1= 4 - l1? I'm trying to solve some exercises from the 2017 IChO, and the problem 6-A1 states that


  in first ionization, an electron with the quantum numbers $n_1 = 4 - l_1$ is removed.


Now, I've never seen this way of expressing quantum numbers, and I don't know what it's supposed to mean, so I would appreciate it if someone could dumb it down to me.
$n_1 = 4 - l_1$ means the same as $n_1 + l_1 = 4$, so the $n$ and $l$ quantum numbers of the first electron added up should equal 4. This could refer to 3p or 4s.

For the second electron removed, the $n$ and $l$ add up to 5. This could refer to 3d, 4p or 5s.

For chromium, the first electron removed is the 4s, followed by the 3d. For copper, this is the case as well, but its mass is higher than that of iron, so it is not the answer.