If you have 1 volume unit ( $u_V$) of water which freezes to ice you'll get $1 u_V + \frac1{11} u_V=\frac{12}{11} u_V$
When the ice is melting to water obviously you'll get again $1 u_V = \frac{11}{11} u_V$. That means the volume of the ice must decrease by $\frac1{12} u_V$