Learn Finnish: Kimppakyyti on halpa, koska jaamme bensarahat. - Carpooling is cheap, because we share the gas money.

It is a compound of kimppa (a colloquial word for doing something together, a pool, or a group) and kyyti (a ride). Together, they form the standard Finnish word for carpooling.

Why is the adjective halpa in the nominative here, rather than the partitive halpaa?

Because kimppakyyti (a carpool or a ride) is a countable, individual concept. If the subject were an uncountable mass noun, you would use the partitive. For example, bensa on halpaa (gas is cheap).

In English, "money" is singular. Why is bensarahat plural here?

While raha (money) is often singular when discussed as a general, uncountable concept, Finnish uses the plural rahat when referring to a specific, complete sum of money meant for a certain purpose, such as paying a specific bill or splitting the exact cost of gas for a trip.

Why is the object bensarahat in the total object case rather than the partitive bensarahoja?

The verb jakaa (to share) takes a total object here because the action is bounded: you are splitting the entirety of a specific cost. Because the noun is plural, the total object takes the -t ending. Remember that plural nouns behave a lot like the personal pronouns you are currently studying (minut, hänet, etc.): they both keep their -t object endings even in passive or necessity sentences, whereas normal singular nouns would drop their ending to become nominative.