What is the mass number of an ion with 109 electrons, 157 neutrons, and a +1 charge?

The mass number can be calculated using the following rule:
mass number = number of protons + number  of neutrons

we have the number of neutrons given as 157 neutrons
now for the protons, we know that in the neutral state, number of protons is equal to the number of electrons. Since this ion has a +1 charge, this means that the ion has one proton more than the number of electrons.
Therefore, number of protons = 109 + 1 = 110 protons

Substituting in the above equation, we can calculate the mass number:
mass number = 110 + 157 = 267

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.

60.0 - there are 3 significant numbers