Answer
$M_{app} = A~m_p$
where $A$ is the mass number and $m_p$ is the proton mass
Work Step by Step
We can say that an atom consists of protons, neutrons, and electrons.
Since electrons have a much smaller mass than protons and neutrons, the mass of the electrons is negligible when we calculate the mass of an atom.
The mass of a neutron is approximately the same as the mass of a proton.
Therefore, we can multiply the total number of nucleons $A$ by the mass of a proton $m_p$ in order to approximate the mass of an atom.
$M_{app} = A~m_p$
where $A$ is the mass number and $m_p$ is the proton mass
