For example Boron has an atomic number of 5. It has a mass number of 10.8. This means that approximately 80% of Boron atoms have a mass of 11 and 20% of Boron atoms have a mass of 10.
The isotope Boron 11
The isotope Boron 10
The mass number is the sum of the protons and neutrons. So to find the number of neutrons subtract the protons from the mass number.