Actually, četrdeset is the word for forty in both Croatian and Serbian; I can't think of a cognate for сорок in either. A good example, however, would be the Croatian word tisuća (= Russian тысяча) and the Serbian equivalent hiljada. The standard Croatian language has always had a tradition of hostility towards foreign borrowings and a tendency to coin Slavic words whenever possible, often by following the examples of other Slavic languages. Thus it often has cognates with Russian where Serbian has adopted various non-Slavic loanwords. However, this word is also a good example of the great difficulty of learning to use such familiar Russian words actively. When I see тысяча in a Russian text, I instantly understand what it means. However, I am completely clueless as to whether it should be stressed тысяча, тысяча, or тысяча, since the Russian stress has no correspondence whatsoever with Croatian even for the most similar words. Furthermore, it's also very hard for me to remember that the word is тысяча, not тысача, тисяча, or тисaча, since Croatian has nothing similar to the Russian concept of hard/soft consonants. Memorizing all this information is almost as hard as memorizing an entirely unfamiliar word.