stylesheet.css

.equation
{
  overflow: hidden;
  position: relative;
  max-width: 400px;
  margin: auto;
  background-color: green;
}
.equationText
{
  width: 70%;
  font-size: 400%;
  text-align: center;
  display: flex;
  justify-content: center;
}
.sumInput
{
  position: absolute;
  width: 30%;
  height: 100%;
  top: 0;
  right: 0;
  background-color: red;
}
.answerText
{
  background: transparent;
  border: none;
  height: 100%;
  font-size: 400%;
}