Question 973255
.
Assuming 0.15cm is the thickness, the volume will be 
volume of the disk with outside radius (r) and thickness (h) 
minus the the inner (empty) disk with radius (r-0.15cm) and 
thickness h:
{{{(pi)r^2h-(pi)(r-0.15cm)^2h}}}=volume of metal in ring
{{{(pi)(2cm)^2(0.5cm)-(pi)(2cm-0.15cm)^2(0.5cm)=volume}}}
{{{(pi)(2cm^3)-(pi)(1.71cm^3)=volume}}}
{{{(pi)(2cm^3-1.71cm^3)=volume}}}
{{{(pi)(0.29cm^3)=volume}}}
{{{0.911cm^3=volume}}}
.
Mass=density x volume:
{{{mass=(21.5g/cm^3)(0.911cm^3)}}}
{{{mass=19.587g}}}