* {
  margin: 0px;
  padding: 0px;
}
#acc {
  width: 800px;
  height: 320px;
  margin: 100px auto;
  border: 1px solid gainsboro;
  overflow: hidden;
}
ul li {
  list-style-type: none;
  float: left;
  width: 160px;
  transition: all 1s;
}
ul:hover li {
  width: 40px;
}
ul li:hover {
  width: 640px;
}
