Learn Polish: Woda to sześćdziesiąt procent masy ciała.

In everyday Polish, the word procent usually stays in this base form after numbers, acting almost like an indeclinable math symbol (%). While procentów (the regular Genitive plural expected after numbers 5 and up) is grammatically correct and sometimes used in formal writing, sześćdziesiąt procent is much more common and perfectly acceptable in daily life.

Why is the word for "mass" in the Genitive case (masy)?

The word procent acts like a noun of quantity. Just as you use the Genitive case after words like "a lot" (dużo) or "a bottle" (butelka), you use it after percentages to express "of." Therefore, sześćdziesiąt procent masy literally means "sixty percent of mass."

Why are there two Genitive words in a row here (masy ciała)?

This is a "chain of Genitives," which Polish uses to express multiple "of" relationships in a row. Masy is Genitive because it follows the percentage (percent of mass), and ciała (from ciało, meaning body) is Genitive because it describes whose mass it is (mass of the body).

Why use to instead of jest for the word "is"?