section img {
  max-width: 100%;
}
.center {
  margin: 0 auto 0 auto;
  max-width: 90%;
  width: 600px;
}
.flex {
  ul {
    display: flex;
    margin: 0;
    padding: 0;
    list-style-type: none;
  }
  li {
    flex: 1 1 auto;
    margin-right: 6px;
  }
  li:last-of-type {
    margin-right: 0;
  }
}