As far as I know, "[number] out of [number]" is another way to express a fraction. Then, would it sound natural to say "one out of five" instead of "one-fifth"?

Native speaker’s answer
Rebecca
This type of smaller denomination or smaller fraction is usually used when we can break down this representation into units, such as people and items. It's difficult to use this expression for pure numbers, such as when trying to represent percentages of money. For example, you could say "one out of five cars sold is an electric car" instead of "one-fifth of cars sold are electric cars," but you couldn't use "one out of X" for money or pure numbers. Below, you will find the most natural ways to express this fraction or percentage. Ex: About a fifth of that goes towards food. Ex: About 20% of that goes towards food. Ex: One out of five people worldwide are at risk of getting COVID-19.