.calculator {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  grid-gap: 8px;
  max-width: 300px;
  margin: 0 auto;
}

.display {
  grid-column: span 4;
  background-color: #f0f0f0;
  padding: 10px;
  font-size: 1.5em;
  text-align: right;
}