Final answer:
The cost for 31 guests at Party B, which has an $850 flat fee for 31 to 50 guests, is simply the flat fee itself, so it's $850.
Step-by-step explanation:
The question asks to determine the cost for 31 guests at a party with a given flat fee structure. Here is how the cost calculation can be made:
According to the information provided, Party B charges an $850 flat fee for hosting a party with 31 to 50 guests. Therefore, since the number of guests in this scenario falls within that range, the total cost for 31 guests would simply be the flat fee itself.
So, the cost for 31 guests would be $850.