We need to calculate the average balance per day.
In this case, the billing period has 31 days and Sandras has an unpaid balance of $720 during 28 days and unpaid balance of $720-$400=$320 during 3 days, so the average balance is:
[tex]\text{average balance=}\frac{(720\cdot28+320\cdot3)}{31}=\frac{21120}{31}=681.29[/tex]