An atom of aluminum has an atomic number of 13 and a mass number of 27. How many neutrons does it have?
The mass number is the total number of protons and neutrons in a nucleus. The atomic number is the number of protons in the nucleus.
With this information it is an easy question:
mass number (p + n) – atomic number (p) = neutrons (n)
\(\rightarrow 27-13=14\) neutrons!