Question

how many neutrons are in one atom of mercury

Answer 1

It depends on the isotope you wanna work with :)

Answer 2

Atomic weight - Atomic number = number of neutrons.
Atomic weight= 201
Atomic number= 80.

Answer 3

your question is certainly wrong...

Answer 4

What if I said that the mass number was 200? ;) (also the isotope with highest natural abundance)
But in general yes just stick to the simple help \(N=A-Z\)
\(N\) is the number of neurons, \(A\) the mass number, and \(Z\) the number of protons.