Question

how many nuetrons and electrons will i have if i have 20 protons and my mass is 40?

Answer 1

The mass number = number of protons + neutrons (the heavier parts of an atom)
The atomic number = number of protons (which also equals the number of electrons)

So,

mass number = 40 = # of protons + # of neutrons
atomic number = 20 = # of protons = # of electrons= #of neutrons

Therefore, the number of electrons is 20.