Machine x makes 300 boxes in 3 mins and machine y makes 300 boxes in 2 mins. with both machines working, how long will it take to make 300 boxes?

-- Machine-X takes 3 minutes for 300 boxes . . . 3/300 = 1/100 minute per box.
-- Machine-Y takes 2 minutes for 300 boxes . . . 2/300 = 1/150 minute per box.

-- 3/150 = 2/100 . . . In 3/150 second, they can make 5 boxes by working together.

-- They need to make 5 boxes (300/5) = 60 times.
-- They'll need to work for 60 times (3/150) minutes.

60 (3/150) = 180/150 = 1.2 minutes = 1 minute and 12 seconds.

What am i doing here. add?