Answer: Greta needs to buy more dog food after 72 days.
Step-by-step explanation: Given that Greta bought three 15-pounds bags of dry dog food and her dog eats 10 ounces of dry dog food each day.
We are to find the number of days after which Greta need to buy more dog food.
We know that

Let Greta needs to buy more dog food after x days.
Then, according to the given condition, we have

Thus, Greta needs to buy more dog food after 72 days.