Lucy packs plates faster she packs 5 plates every minute. Mike packs 4 plates every minute because 3/4 of an hour is 45 minutes, then you divide 180 by 45 and the answer is 4 per minute.
Franco would have to pack 90 plates every 20 minutes (1/3 of an hour) for his rate to be between Lucy's and Mike's. Lucy's rate is at 100 plates per 20 minutes because she can pack 5 plates every minute. Mike's it's at 80 plates per 20 minutes because he can pack 4 plates per minute. So that means that For Franco to be in the middle he has to pack 90 Plates every 20 minutes.
Hope this helps!!!