Breakpoint mixin

breakpoint_mixin.scss

@mixin breakpoint($point) {
  @if $point == large {
    @media (min-width: 64.375em) { @content; }
  }
  @else if $point == medium {
    @media (min-width: 50em) { @content; }
  }
  @else if $point == small {
    @media (min-width: 37.5em)  { @content; }
  }
}

output.css

.page-wrap {
  width: 75%;
}
@media (min-width: 64.375em) {
  .page-wrap {
    width: 60%;
  }
}
@media (min-width: 50em) {
  .page-wrap {
    width: 80%;
  }
}
@media (min-width: 37.5em) {
  .page-wrap {
    width: 95%;
  }
}

usage.scss

.page-wrap {
  width: 75%;
  @include breakpoint(large) { width: 60%; }
  @include breakpoint(medium) { width: 80%; }
  @include breakpoint(small) { width: 95%; }
}