Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
strong [strɔŋ] adj си́льный*силён (healthy, powerful) кре́пкий*кре́пок (object, material) про́чный*про́чен (imagination) большо́й; (drugs, chemicals) си́льный#; (letters, measures) ре́зкий*ре́зок
adv
to be going strong занима́ть (impf) про́чные пози́ции
they are 50 strong их 50
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.