LATIN 1Z03 : An Odd Use of "Cum"

Arrange the following groups of atoms in order of increasing first ionization energy. (Use the appropriate <, =, or > symbol to separate substances in the list.)

a) Be, Rb, Na

b) Se, Se, Te

c) Br, Ni, K

d) Ne, Sr, Se

a. 35 b. 45 c. 46 d.70 e. 81. The answer is 46. But I got 47 in my calculation somehow. Since Mg is 2+, I looked at all the elements that has a 2-, which are O, S, Se, Te, Po. Since the question said that its mass is 81, i chose Se since it's the closest to it. Se has atomic number of 34, meaning it has 34 protons. Mass number is from proton and neutron. So I subtracted 81-34: 47. Why is it 46??