For the RU-EN sentence - yes, 6 is accepted; for the EN-RU, however, it's not. Numerals are accepted in (almost) all English translations, since Duolingo's algorithm was taught to accept them. However, users (in particular those who are not English speakers) should take the (little) extra time to write numbers in letters, since they also have to do it when they write in Russian. Many users of this course are not anglophones, so having to write numbers in letters cannot be bad for them, either in English or in Russian, anyway... So I'd suggest writing everything in letters only.