No explanation: one in five = uno/a su cinque. Cheer up, how do you think people struggling with two foreign languages at a time are doing? In this case both Italian and English are foreign languages. They have to understand what "su" means and then find the right translation in English. I never told anybody that learning a language is easy. It is time consuming, often tedious but most of all it is fun because you will get a different view on how to say things and will be able to communicate in another language.
"One (out) of five", or "one fifth", or "one in five" can be taken to have the same meaning (subject to context). So the English translation isn't strictly literal in this case.
"Uno di cinque" could be another way of saying it, although a native speaker could tell you which is the commonly used approach.