I don't think that you should be trying to make the most CD players as possible because one cd player brings the most profit. Remember, 1 CD Player takes 5 hours to make and brings in $7 profit. 1 Casette player takes 2 hours to make, and brings in $4 profit. But if to make 2 Casette players, it takes 4 hours, and you get a profit of $8 and you still have 1 hour left over i.e. making two catsette players gives you $1 more profit than making one cd player, and it takes less time, a whole hour less. so what you do is that you see how many cassete players you can make, because that will maximise profit.

750*2 = 1500 hours taken to make the max 750 cassette players a month.

3000-1500 = 1500 hours left to make cd players.

1500/5 = 300 CD Players a month.

Total Profit = 1500*4 + 300*7 = $8100

I hope i'm right .