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

This is how you would normally write it out:

A (mass number which is the sum of protons and neutrons)

X (atomic name)

Z (atomic number which is the proton)

That being the case, we can go ahead and fill this out with what's given,

20

Ne

10

The question is asking for the amount of neutrons in this element so take the 20 containing both protons and neutrons and subtract that by the given atomic number (aka protons) to get 10 neutrons.