This question contains a mistake. Atomic number determines the number of protons in an atom, and the atomic mass is the sum of protons and neutrons. The number 45 you provided is likely the atomic number, not the atomic mass.

Here's how to calculate the number of neutrons:

* Atomic mass = Number of protons + Number of neutrons

* Number of neutrons = Atomic mass - Number of protons

Since the atomic mass is 20 and the atomic number (number of protons) is 45, we can calculate the number of neutrons:

* Number of neutrons = 20 - 45 = -25

A negative number of neutrons is not possible. This means there's likely an error in the information provided. It's impossible for an atom to have an atomic mass of 20 and an atomic number of 45.

Please double-check the information you provided, or clarify which value is the atomic number and which is the atomic mass.