Chapter 2 - Atoms, Molecules, and Ions - Questions & Problems - Page 73: 2.110

Please see the work below.

We know that $Mass=neutron(n)+protons(p)$ but given that $n=1.2p$ hence $Mass=1.2p+p=2.2p$ $55=2.2p$ $p=25$ Now $neutrons= 1.2\times 25=30$ and $mass=30+25=55$ Thus the required element is Manganese denoted by Mn.