Flexbox

.container {
  display: flex;
}
.container > div {
  padding:30px;
}
#first {
  background-color: teal;
  flex: 1 1 auto;
}
#second {
  background-color: orange;
}

.container {
  display: flex;
}
.container > div {
  padding: 30px;
}
#first {
  background-color: teal;
  flex: 0 1;
}
#second {
  background-color: orange;
}

.container {
  display: flex;
}
.container > div {
  padding: 30px;
}
#first {
  background-color: teal;
  flex: 0 100px;
}
#second {
  background-color: orange;
}

.container {
  display: flex;
}
.container > div {
  padding: 30px;
}
#first {
  background-color: teal;
  flex: 2;
}
#second {
  background-color: orange;
  flex: 1;
}

.container {
  display: flex;
}
.container > div {
  padding: 30px;
}
#first {
  background-color: teal;
  flex: 50%;
}
#second {
  background-color: orange;
  flex: 50%;
}