<div class="parent">
  <div class="child">a</div>
  <div class="child">b</div>
  <div class="child">c</div>
  <div class="child">d</div>
  <div class="child">e</div>
  <div class="child">f</div>
  <div class="child">g</div>
</div>

.parent {
  margin: 20px auto;
  display: flex;
  flex-flow: row wrap;
}

.child {
  min-height: 100px;
  background: green;
  border: 5px solid white;
  flex: 1 1 auto;
  line-height: 100px;
  text-align: center;
}