<div class="box">
    <div class="box-item _side">1</div>
    <div class="box-item _main">2</div>
</div>
.box { 
  display: flex; 
  background: #eee;
}
.box-item{
  background: red;
  height: 50px;
  margin:10px;
}
._main{
  width: 100%;
  flex:1;/*←これ 伸縮率指定しないとIE11でwidth:100%が効かない*/
}
._side{
  min-width: 50px;/*←これ*/
  width: 50px;/*←これ*/
}

External CSS

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External JavaScript

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