How do I work out the number of electrons without an atomic number?

I have been given a question for my level 3 pharmacy test giving me the atomic mass of an atom and the number of neutrons, and asking how many electrons this atom has. I have searched high and low for a way of calculating this but everything I come across says you need an atomic number to work it out, which I don't. There must be a way as when I selected answer 'It's impossible to tell', it came back from the examiner as incorrect.