How would you give the nuclear symbol for the isotope of gallium, which contains  40 neutrons per atom?

A nuclide of an element is expressed as; ${}_{\mathrm{Z}}{}^{\mathrm{A}}\mathrm{X}$. Where 'Z' is the atomic number and 'A' is atomic number. 

We know Gallium is an element having atomic number 31. The mass number of an atom is the same as the sum of the number of protons and neutrons. 

Given it has 40 neutrons, the mass number becomes; $40+31=71$.

Thus the symbol is; ${}_{31}{}^{71}\mathrm{Ga}$.