.flex-container {
  /* We first create a flex layout context */
  display: flex;
  
  /* Then we define the flow direction 
     and if we allow the items to wrap 
   * Remember this is the same as:
   * flex-direction: row;
   * flex-wrap: wrap;
   */
  flex-flow: row wrap;
  
  /* Then we define how is distributed the remaining space */
  justify-content: space-around;
  
  padding: 0;
  margin: 0;
  list-style: none;
}

.flex-item {
  background: Ivory;
  padding: 5px;
  width: 500px;
  height: 400px;
  margin-top: 10px;
  line-height: 25px;
  color: white;
  font-weight: bold;
  font-size: 1.15em;
  text-align: center;
}

.flex-item-exercice {
  background: Ivory;
  padding: 5px;
  width: 250px;
  height: 175px;
  margin-top: 10px;
  line-height: 25px;
  color: white;
  font-weight: bold;
  font-size: 1.15em;
  text-align: center;
}

.flex-item-delivery {
  background: Ivory;
  padding: 5px;
  width: 250px;
  height: 60px;
  margin-top: 10px;
  line-height: 25px;
  color: white;
  font-weight: bold;
  font-size: 1.15em;
  text-align: center;
}