flex-grow

/* <number> valores */
flex-grow: 3;
flex-grow: 0.6;

/* Valores globales */
flex-grow: inherit;
flex-grow: initial;
flex-grow: unset;

<h4>This is a Flex-Grow</h4>
<h5>A,B,C and F are flex-grow:1 . D and E are flex-grow:2 .</h5>
<div id="content">
  <div class="box" >A</div>
  <div class="box" >B</div>
  <div class="box" >C</div>
  <div class="box1" >D</div>
  <div class="box1" >E</div>
  <div class="box" >F</div>
</div>

#content {
  -ms-box-orient: horizontal;
  display: -webkit-box;
  display: -moz-box;
  display: -ms-flexbox;
  display: -moz-flex;
  display: -webkit-flex;
  display: flex;

  -webkit-justify-content: space-around;
  justify-content: space-around;
  -webkit-flex-flow: row wrap;
  flex-flow: row wrap;
  -webkit-align-items: stretch;
  align-items: stretch;
}

.box {
  flex-grow: 1;
  border: 3px solid rgba(0, 0, 0, 0.2);
}

.box1 {
  flex-grow: 2;
  border: 3px solid rgba(0, 0, 0, 0.2);
}