Learn Russian: Это справедливый план: мы делим взносы поровну. - It is a fair plan: we divide the contributions equally.

You could rephrase the idea to say "each pays as many contributions as the other" using this lesson's grammar: каждый платит столько же взносов, сколько и другой. However, using the single adverb поровну is the most natural and concise way to say a group is splitting something "equally" or "in equal parts".

How does the adverb "поровну" differ from other words for "equal", like "наравне"?

Поровну specifically means "in equal parts" and is almost always used with verbs about dividing, sharing, or splitting (like делить here). Наравне means "on an equal footing" or "on par with", used when doing things alongside others, like working or competing equally.

What case is "взносы" in this sentence, and how can you tell?

It is in the accusative case, plural form, because it is the direct object receiving the action of делим (we divide). Because взнос (contribution, fee, or dues) is an inanimate masculine noun, its accusative plural form is identical to its nominative plural form.

Is "справедливый" the only way to say "fair" in Russian?

It is the right word for "fair" when you mean just, equitable, or objective, such as a fair plan or a fair trial. If you mean "fair" in other English senses, you need different words: светлый for fair hair, or честный for playing fair (honestly).