html {
  box-sizing: border-box;
}

*, ::before, ::after {
  box-sizing: inherit;
}

/* Opmaak van de boxes */
.flex-container > div {
  border: 1px solid #000;
  background-color: lightgreen;
  padding: 10px;
  flex-grow: 1;
}

/* Plaats je oplossing hieronder */
.flex-container{
  display: flex;
}

.flex-container>div:first-child, .flex-container>div:last-child{
  flex-grow: 2;
}