Milly is selling Girl Scout cookies. She sells 8 boxes every 3 days. How many days will it take her to sell 56 boxes?(1 point)

Let's calculate the number of sets of 3 days it would take to sell 56 boxes of girl scout cookies: 56 boxes / 8 boxes per 3 days = <<56/8=7>>7 sets of 3 days.

Since each set is 3 days, it would take her 7 * 3 = <<7*3=21>>21 days to sell 56 boxes. Answer: \boxed{21}.