Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
reasonably [ˈriːznəblɪ] adv (sensibly) разу́мно; (fairly) дово́льно
one can reasonably assume that ... мо́жно справедли́во предположи́ть, что ...
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.