#### Answer

See answer below.

#### Work Step by Step

x is the mass percentage of CaO.
$2/1\times 7.695\ g\times x\div 56.08\ g/mol+ 2/1\times 7.695\ g\times (1-x)\div 40.30\ g/mol=0.125\ L\times 2.55\ M$
$0.274x+0.382(1-x)=0.319$
$x=0.5875=58.75\%$
MgO: $41.25\%$