i have confusion that when a question is given how to know whether to find h.c.f or l.c.m
To know whether to find h.c.f. or l.c.m. first read the question carefully and look for what quantities are to be determined.
If the problem asks for maximum things, like find the maximum numbers of rooms required, greatest size of a measuring can, etc., then find HCF.
If the problem asks for simultaneous reoccurrence of events then find LCM.