Learn Russian: Мамин билет на сто рублей дороже, она переплатила. - Mom's ticket is a hundred rubles more expensive, she overpaid.

When expressing by how much something is greater or lesser (the comparative of difference), Russian uses the preposition на followed by the accusative case. Here, на сто рублей literally means "by a hundred rubles," showing the exact difference in price before the comparative дороже (more expensive).

Why is it сто рублей if it is supposed to be in the accusative case?

The numeral сто (one hundred) has the same form in both the nominative and the inanimate accusative cases. Since рубль (ruble) is inanimate, the accusative phrase на сто рублей looks exactly like the nominative. The noun рублей is in the genitive plural because it follows the number 100.

Why is it Мамин билет instead of Билет мамы?

Both are correct, but Russian often uses possessive adjectives ending in -ин (for feminine/a-declension nouns like мама) or -ов/-ев (for masculine nouns) when talking about family members. Мамин acts like a regular adjective modifying билет, which sounds very natural and slightly more conversational than using the genitive мамы.

What does the prefix пере- add to the verb in переплатила?