The answer to this question would be 22 neutrons.
In this question, you are given an atom which has 42 mass number and 20 atomic number. Mass number indicate the atom mass. Atom mass is made by neutrons and protons since electron mass is not significant. Atomic number indicates how much proton the atom has. Since the mass is 42 and 20 of it from the proton, then the neutron in the atom is 42 - 20 = 22 neutrons.
