The number of neutrons in dipositive zinc ion with mass number 70 is

Number of neutrons = Mass number - Atomic number = 70-30 = 40.

Dipositive zinc is Zn2+. Atomic weight of Zn is 70, atomic number of Zn is 30.

Number of neutrons:

$$\begin{gathered} \mathrm{Number} \mathrm{of} \mathrm{neutrons}=\mathrm{Mass} \mathrm{number}-\mathrm{number} \mathrm{of} \mathrm{protons} \\ \mathrm{Number} \mathrm{of} \mathrm{neutrons}=70-30=40 \end{gathered}$$