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<div class="parent">
  <div class="child"></div>
  <div class="child"></div>
  <div class="child"></div>
</div>
.child {
    /*Custom*/
    width: 30%;
    margin-right:3%;
    height: 200px;
    float:left;
    background: #005b7f;
    /*Custom*/
    opacity: 1;
    -webkit-transition: all 300ms ease;
    -moz-transition: all 300ms ease;
    -o-transition: all 300ms ease;
    transition: all 300ms ease;
}
.parent:hover .child:not(:hover) {
    opacity: 0.2;
}