Milly is selling Girl Scout cookies. She sells 8 boxes every 3 days. How many days will it take her to sell 56 boxes?
Milly sells 8 boxes every 3 days so she sells 8/3 = <<8/3=2.6666666666666665>>2.67 boxes per day
She wants to sell 56 boxes. If she sells 2.67 boxes a day then it will take her 56/2.67 = <<56/2.67=20.97518690973783>>20.98 days to sell them all
It will take her 20.98 days to sell 56 boxes. She can't sell .98 of a day so it will take her <<20=20>>20 full days. Answer: \boxed{20}.