Answer:
They will both owe their parents $200 when Casey is paying his 5th month and Sally is paying on her 5th month.
Step-by-step explanation:
If Casey pays 60 dollars after 5 months, he will have payed $300 dollars. He still needs to pay $200.(500-300) If Sally pays 80 dollars after 5 months, she will have payed $400 dollars. She still needs to pay $200. (600-400)