@nitin A=mass number is sum of the number protons and neutrons in an atom.
Z= Atomic Number is the number electron/protons present in an atom(netrual atom will have the same number of protons and electrons). Then to find the number of neutrons:
Mass Number – Atomic Number = No. of Neutrons
70 – 30 = 40 in Zinc atom
No. Neutrons = 40