Final answer:
Brian's average bill over September, October, and November is $300.
Step-by-step explanation:
To find Brian's average bill over the three months, we need to calculate the total bill for the three months and then divide it by 3. In September, the bill was $200. In October, it was $200 + $50 = $250. In November, it was $250 + $200 = $450. The total bill for the three months is $200 + $250 + $450 = $900. So, the average bill over the three months is $900 รท 3 = $300.