How many neutrons does element X have if its atomic number is 39 and its mass number is 75?

The answer is 36.

An atom usually contains protons, neutrons and electrons. Atomic number means the number of protons in the atom, while mass number is the sum of protons and neutrons of the atom.

Therefore, in order to find the number of neutrons, we just need to subtract the number of protons from the mass number.

75 - 39

= 36

The number of neutron is 36.