different russian numerals have very different types of declension. the word (one) is declined by number, (in singular) gender and case. the word (two) is declined by gender and case, all other numbers have only case to decline by. the words for 50, 60, 70, 80, 200, 300, 400, 500, 600, 700, 800, 900 are unique for russian, as they decline not only with ending in their end, but also with part of word in their middle (since they are originally composed from two words): nom. (50) – gen. etc. (compare – "five tens").