Edit in JSFiddle

<div class="parent">
  <div class="left">left</div>
  <div class="right">right</div>
</div>

@mixin common{
  line-height: 100px;
  height: 100px;
  text-align: center;
  color: white;
}
.parent{
  @include common;
  display: flex;
  .left{
    background-color: red;
    width: 100px;
  }
  .right{
    background-color: blue;
    flex: 1;
  }
}