How to Count in Russian рубли (rubles):
In Russian, the word рубль (ruble) changes its form depending on the number that comes before it. Think of it as having three simple rules:
1 рубль (1, 21, 31, 101...) Nominative singular.
Example: 1 рубль, 21 рубль, 31 рубль and so on.
2, 3, 4 рубля́ (2 - 4, 22 - 24, 102 - 104...) Genitive singular.
Example: 2 рубля́, 3 рубля́, 4 рубля́ and so on.
5 to 20 рубле́й and numbers ending in 0, 5–9 (5, 11–20, 25, 100...) Genitive plural.
Example: 5 рубле́й, 10 рубле́й, 100 рублей
Quick Rule of Thumb: Look at the last digit of the number!
Ends in 1 - рубль
Ends in 2, 3, 4 - рубля́
Ends in 5–9, 0, or teens (11–19) - рубле́й