An element has atomic number 10 and atomic mass 20. How many neutrons does this element have?
The atomic mass represent the number of protons and neutrons in the nucleus of the atom. If you subtract the number of protons to the atomic mass, you’ll get the number of neutrons!
Z = 10 ("Z" represents the number of protons)
A = 20 ("A" represents protons + neutrons)
Number of neutrons is (A - Z):
Neutrons = 20 - 10 = 10 neutrons.