The ion Aʳ⁺ has mass number q and atomic number p. How many neutrons does it have?

Correct answer: b q − p

Neutrons = A − Z = q − p; the charge r does not change the neutron count.

Why the other options are wrong

• a p: p is the atomic number, the proton count; neutrons = mass number − protons.
• c q: q is the mass number, protons plus neutrons; the neutrons are fewer.
• d p − r: p − r is the electron count; the charge is not needed for neutrons.