In order to calculate an average for a list of numbers, file sizes, durations, or range of numbers, or a table with columns containing numeric values you can use the math avg command.
This command is part of the math module of Nushell.
When the input is a list then the result is a single value with the average of all values in the list.
If you use a table as input the result is a record where the key is the column name and the value the average of all values in that column.
Finally it is possible to have a single value as input and the result is the same value obviously.