Edit in JSFiddle

<div class="demo1">
  <div class="left">定宽</div>
  <div class="right">自适应</div>
</div>

@mixin common{
  line-height: 100px;
  height: 100px;
  text-align: center;
  color: white;
}
.demo1{
  @include common;
  .left{
    background-color: red;
    float: left;
    width: 100px;
  }
  .right{
    margin-left: 100px;
    background-color: blue;
  }
}