Mean, Median, Mode & Range
Mean is the sum divided by the count; median is the middle value of an ORDERED list (average the two middle values if the count is even); mode is the most frequent value; range is maximum minus minimum.
A data set can have no mode, one mode, or several. For ‘find the missing score’ problems, multiply the target mean by the number of values to get the required total, then subtract the known values.