A sample of ancient bone unearthed in an archeological dig has 10 g of carbon in it. It is found to have a 14C activity of 50 decays per minute. How old is the bone sample

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.