.container {
  width: 180px;
  margin: 50px auto;
  border: 2px solid black;
}

.output {
  width: 180px;
  height: 50px;
  line-height: 50px;
  text-align: right;
  padding: 5px;
  margin-bottom: 5px;
  border:2px solid black;
  box-sizing: border-box;
  display:none;
}

.output.active {
  display:block;
}

.input {
  width:180px;
  border: 2px solid black;
}

.row {
  display: flex;
  justify-content: space-between;
  border: 1px solid red;
  width: 180px;
}
/*rowの範囲がわかりやすいように赤の枠を作っています。*/

.num_bth, #clear, #equal_btn{
  width: 60px;
  height: 30px;
  text-align: center;
  line-height: 30px;
  border-radius: 30%;
  background: lightgray;
  cursor: pointer;
  margin: 5px;
}