Learn Russian: Я забыл о пятидесяти купюрах в кошельке. - I forgot about the fifty banknotes in the wallet.

The phrase забыть о (to forget about) requires the Prepositional case. Declining numbers like 50, 60, 70, and 80 can be tricky because both halves of the word change. Since the Prepositional form of пять is пяти and десять is десяти, they combine to form пятидесяти.

Shouldn't nouns after numbers like 50 be in the Genitive plural? Why is it купюрах instead of купюр?

The 'Genitive plural after 5 and up' rule only applies when the number is in the Nominative or Accusative case (for example, пятьдесят купюр). When a preposition forces the number into an oblique case—here, the Prepositional case because of о—the noun simply matches that case. So, both the number and the noun take the Prepositional plural: о пятидесяти купюрах.

Why did the ё disappear in кошельке?

The dictionary form is кошелёк (wallet). This word contains a 'fleeting vowel' (the ё), which drops out when you add case endings. When we put it in the Prepositional case to say 'in the wallet' (в + Prepositional), the ending -е is added, and the ё disappears, resulting in в кошельке.

Why do we use о here instead of об or обо?