In your response, you mention the property of mean as the impact of increasing the number of observations while keeping their sum the same. However, this is not entirely accurate. When you add a new observation of zero to the set of numbers, the sum of the observations will increase by zero, which means it will remain unchanged. As a result, the effect on the mean will depend on the original set of numbers and their average.
To correctly answer the question about the effect of the zero on Joey's quiz average, you should consider the impact of this new observation in relation to the original set of numbers. Since zero is a value that lies below the average, adding it to the set will generally pull down the average. Therefore, a correct answer could be:
"The addition of zero to Joey's quiz average lowered the mean because zero is a value below the original average, causing a decrease in the overall average."