title: Simplifying Boolean Expression x'yz + xy'z + xyz' + xyz
body: <p>Hi I'v solved this halfway, please help me for the rest.<br/>
  so far I've got..</p>
  
  <pre><code>    x'yz + xy'z + xyz' + xyz
      z(x'y + xy') + xy(z'+z)
      z(x'y + xy') + xy
  </code></pre>
  
  <p>i don't understand how to solve the <b><em>z(x'y + xy')</em></b> part of this expression.. please somebody help..</p>