body {
  margin: 20px;
  padding: 0;
}
* {
  box-sizing: border-box;
}
.column-1-7 {
  width: calc(100%/7);
  background-color: #c2e3ec;
}
.column-2-7 {
  width: calc(2*100%/7);
  background-color: #c2ece0;
}
.column-3-7 {
  width: calc(3*100%/7);
  background-color: #c2ceec;
}
.col {
  float: left;
  height: 500px;
  border: 1px solid black;
}