11.1 flex - flex固定左宽固定，右宽自动

html

<div class="wrap">
    <div class="lbd">左侧</div>
    <div class="rbd">右侧</div>
</div>

css

.wrap{
  display: flex;
  .lbd{
    flex: 0 0  100px;
  }
  .rbd{
    flex: 1;
  }              
}