so, we'll end up with two parts, one being 4 times larger than the other, namely if the other is 1 slice, then the large part is 4 slices, meaning both parts will be on a 4:1 ratio.
well, simply enough, we can simply split 90 in (4+1) pieces, and then distribute accordingly to each part
[tex]\bf \cfrac{\textit{large part}}{\textit{small part}}\qquad \qquad \stackrel{ratio}{4:1}\qquad \cfrac{4\cdot \frac{90}{4+1}}{1\cdot \frac{90}{4+1}}\implies \cfrac{4\cdot 18}{1\cdot 18}\implies \boxed{\cfrac{72}{18}}[/tex]
