Learn Russian: Я приготовил полторы порции риса. - I prepared one and a half portions of rice.

The word for 1.5 changes based on gender. Because the noun it counts, порция (portion), is a feminine noun, we use the feminine form полторы. If we were counting a masculine or neuter noun, like стакан (glass), we would use полтора (полтора стакана).

Why does порция become порции?

When полтора or полторы is in the nominative or accusative case (as it is here, being the direct object), the noun being counted must be in the genitive singular. The genitive singular of the feminine noun порция is порции.

Wait, isn't the fraction the direct object? Why doesn't полторы change its ending for the accusative case?

It actually is in the accusative case! For the numbers meaning 1.5, the accusative case is identical to the nominative case when you are talking about inanimate objects (like portions of food). It will only change its form when used in oblique cases like the dative, instrumental, or prepositional.

Why is the word for rice (рис) also in the genitive case (риса)?

It acts just like the English word "of". You prepared portions of rice. When one noun describes the makeup, contents, or owner of another noun, it takes the genitive case.