When a company makes 50 deliveries in 5 days, we can first determine the amount of deliveries in 1 day. This is 50 packages / 5 days = 10 packages per day. Hence, the company delivers 10 packages a day.
400 deliveries will therefore take them:
400 packages / 10 packages a day = 40 days
Hence, the anser is 40 days.