Set width of the right div based on left div

Question

I have a markup where I have a container of lets say width 100% and I have two child elements in this container. The first child has a width based on the text inside of this div then the next div should have a (width of the container's width) - (the first child element).

See image below for clarification:

In this case the white box has a width of 100% and has a width based on its text content. The green div should have the (width of white box) - (red box width).

How do I do set the green width in CSS? This is what I have tried so far:

.ContentItemMenu {
  width: 100%;
  height: 50px;
  /* background-color: bisque; */
  border-bottom: 1px solid #c3c3c3;
}

.ContentTitle {
  width: auto;
  padding: 0 20px;
  height: 100%;
  float: left;
  font-size: 25px;
  font-family: opensansBold;
  text-align: left;
  line-height: 50px;
  background-color: #5144F0;
}

.SubContentTabs {
  width: 100%;
  height: 100%;
  float: left;
  background-color: aquamarine;
}

<div class="ContentItemMenu">
  <div class="ContentTitle">Some text</div>
  <div class="SubContentTabs"></div>
</div>

I am a complete newb to css, so if more clarification is needed let me know.

Answer 1

Use a flexbox - add display: flex to the ContentItemMenu and set flex: 1 to SubContentTabs - see demo below:

.ContentItemMenu {
  display: flex; /* ADDED */
  width: 100%;
  height: 50px;
  /* background-color: bisque; */
  border-bottom: 1px solid #c3c3c3;
}

.ContentTitle {
  width: auto;
  padding: 0 20px;
  height: 100%;
  float: left;
  font-size: 25px;
  font-family: opensansBold;
  text-align: left;
  line-height: 50px;
  background-color: #5144F0;
}

.SubContentTabs {
  width: 100%;
  height: 100%;
  float: left;
  background-color: aquamarine;
  flex: 1; /* ADDED */
}

<div class="ContentItemMenu">
  <div class="ContentTitle">Some text</div>
  <div class="SubContentTabs"></div>
</div>
<br/>
<div class="ContentItemMenu">
  <div class="ContentTitle">Some text here, this is a long text</div>
  <div class="SubContentTabs"></div>
</div>