The two cities are actually 427.5 miles apart.
The formula of an area of a square: 
a - side
We have: 
substitute:

The fromula of a perimeter of a square is: 
substitute:

1. Find the unit rate
15 books divided by 5 days = 3 books sold per day.
Since we are looking for how many days it would take the store to sell 21 bucks, we could do 21 (amount of books) divided by 3 (how much books is sold in a day).
Therefore, it will take a week / 7 days to sell 21 books.
1/3
Reduces los números usando el máximo común divisor= 2