<div class="flex">
  <div class="item-a">flex-grow: 1;</div>
  <div class="item-b">flex-grow: 9999;</div>
</div>

.flex {
  display: flex;
  flex-wrap: wrap;
  
  > * {
    display: flex;
    align-items: center;
    justify-content: center;
    box-sizing: border-box;
    height: 240px;
    font-weight: 700;
    font-family: arial, sans-serif;
    color: #222;
  }
}
/* コンテナーのブレイクポイント
　　240 + 400 = 640px */
.item-a {
  flex: 1 1 240px;
  background: #faa;
}
.item-b {
  flex: 9999 1 400px; 
  background: #aea;
}