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.block {
  width: 200px;
  height: 60px;
  background: black;
  position: relative;
  
  &:after {
    border-left: 15px solid transparent;
    border-right: 15px solid transparent;
    content: "";
    position: absolute;
    top: 140px;
    left: 50%;
    width: 0; 
    height: 0;
  }
}