Hi Vijay,

Mass number = Number of protons + Number of neutrons

Atomic number = number of protons or number of electrons

Atomic number of zinc = 30 i.e., has 30 electrons.  In dipositive ion 2 electrons are lost, so, 28 electrons are present.

Mass number = 70

70 = 30 + Number of neutrons

Thus number of neutrons in dipositive zinc ion with mass number 70 is 40.

Feel free to ask doubts in the Comment Section.

I hope this information helps you.

Good Luck!