Rashed can stuff advertising circulars into envelopes at the rate of 45 envelopers per minute and Reena requires a minute and a half to stuff the same number of envelopes. Working together, how long will it take Rashed and Reena to stuff 300 envelopes?

in 60 seconds Rashed can stuff 45 envelope  in 1 seconds Rashed can stuff 45/60=3/4 envelope   again, in 90 seconds Reena can stuff 45 envelope in 1 seconds Reena can stuff 45/90=1/2 envelope

Together in one second they can stuff=(3/4+1/2)=5/4 envelope they can stuff 5/4 envelope in 1 second they can stuff 1 envelope in 1X4/5  second they can stuff 300 envelope in 1X4X300/5  second=240 second=4 minutes