This math topic focuses on advanced problems related to calculating the mean of a set when a value is missing. Practicing skills include determining what number should replace the missing value (denoted as '?') in a series to achieve the specified average. Each problem provides a series of numbers with one value missing and a stated mean. Students must use their understanding of mean calculation to find the correct missing number from multiple choices provided. This involves multiplying, adding, and reasoning to back-calculate the missing value that results in the given mean.