How many neutrons does element x have if its atomic number is 37 and its mass number is 80?

43.

Explanation:

Mass number is the sum of protons and neutrons of an atom.

The number of prortons is the atomic number, so you can write:

Mass number = atomic number + number of neutrons. From which you get;

number of neutrons = mass number - atomic number = 80 - 37 = 43

All the atoms of a given element have the same number of protons. That is why the number of protons idenfies the elements and is called atomic number.

The number of neutrons of a given element may be different for different atoms of the same element.

The atoms of an element that have different number of neutrons are called isotopes. So, different isotopes of the same element have same atomic number but different mass number.