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SassMeister-input.scss

// fiddling with auto conversion between rem and px

$base-font-size: 10px;

@function rem($n) {
    @if unitless($n) {
        @return $n * 1rem;
    }
    @else if unit($n) == 'px' {
        @return $n / $base-font-size * 1rem;
    } @else {
        @return $n;    
    }
}

$mobile: 48rem; //480px
.test {
    @include breakpoint($mobile) {
        width: rem(100px);
    }
}

SassMeister-output.css

@media (min-width: 48rem) {
  .test {
    width: 10rem;
  }
}