# What element has a mass number of 55?

On the periodic table, manganese is element number $24$, and has an average atomic mass of $54.938 \setminus \text{amu}$, and we can round that off to $55 \setminus \text{amu}$.
So, I'd say that the element with a mass of $55 \setminus \text{amu}$ is manganese $\left(M n\right)$.