An element has atomic number 10 and an atomic mass of 20. how many neutrons are in the atom of this element

There are 10 neutrons in this atom of an element.

Explanation:

Atomic mass of an element  = 20

Atomic mass number is the sum of number of protons and neutrons that are present in an atom.

Atomic Mass number = Number of protons + Number of neutrons

Atomic number of an element = 10

Atomic number is the number of protons or number of electrons that are present in an atom.

Atomic number = Number of electrons = Number of protons

Number of protons = 10

Atomic Mass number = Number of protons + Number of neutrons

20 = 10 + Number of neutrons

Number of neutrons = 10

There are 10 neutrons in this atom of an element.