For rem value of font-size mixin, first divide $sizeValue by 1.6 ...

...since in browsers, 1rem is equal to 16px

if 1.6 is passed, we'd expect 1rem (1.6/1.6) = 1rem
if we passed 2.4 expecting 24px, we end up with 1.5 rem. 16 * 1.5 = 24px.

Old method would essentially add:
font-size: 16px;
font-size: 25.6px (because 1.6 rem = 16px * 1.6)