Fixed width div on left, fill remaining width div on right

.header {
  float: left;
  background: #efefef;
  background-repeat: no-repeat;
  width: 240px;
  height: 100px;
}

.header-right {
  overflow: hidden; 
  background-color: #000;
  height: 100px;
}

<div class="header"></div>
<div class="header-right"></div>