Mass number will always give the idea of how many protons and neutrons its nucleus contains.
Mass number = no. of protons + no. of neutrons n atom of calcium, Ca , has 20 neutrons in its nucleus.
Therefore, the mass number of this calcium isotope will be
mass number = A = 20 protons + 20 neutrons = 40
This will be calcium-40 isotope, 40 Ca 20