01.css-media-print.css

.a4-page{
  padding: 0 50px;
}
.a4-page+.a4-page{
  border-top: 1px solid #333;
}
.page-title{
  margin: 20px 0;
  text-align: center;
  font-size: 1.2rem;
}
.ans-outer{
  text-align: right;
  margin-bottom: 20px;
}
.ans-area{
  display: inline-block;
  width: 4.5em;
  border-bottom: 1px solid #333;
  text-align: left;
}
.ans-area::before{
  content: "A. ";
}
.hide{
  display: none;
}
@media print{
  body{
    height: 200%;
  }
  #a4-outer{
    width: 100%;
    height: 100%;
    position: absolute;
    top: 0;
    left: 0;
    letter-spacing: 0;
    font-size: 0;
  }
  .a4-page{
    display: flex;
    flex-direction: column;
    box-sizing: border-box;
    height: 100%;
    padding: 10mm 20mm;
    background-color: white;
    border-bottom: 0;
    letter-spacing: 2px;
  }
  .a4-page+.a4-page{
    border: none;
  }
  .page-title{
    margin: 0;
    flex-grow: 2;
    font-size: 24px;
  }
  .que,.ans-outer{
    flex-grow: 1;
    font-size: 20px;
  }
}

02.css-media-print.html

<button id="disp-ans">解答を表示</button>
<div id="a4-outer">
  <div class="a4-page" >
    <!-- コンテンツ(今回はjsで生成) -->
  </div>
  <div class="a4-page" >
    <!-- コンテンツ(今回はjsで生成) -->
  </div>
</div>