sorry if this isn't helpful (i don't know how to write in the pseudocode that SDD requires well but)
a series would probably be an array, then use a FOR loop to add each item in that array, and divide by array length?
(i'll try write it in the pseudocode NESA provides but this is quite literally my first try
)
BEGIN AVERAGECALCULATOR read(series)
let seriesLength = 0
let total = 0
REPEAT
get seriesitem
total += seriesitem
seriesLength +=1
UNTIL end of series
output total/seriesLength
END AVERAGECALCULATOR
(sorry if this isn't really good, just wanted to give it a try)
(edit: tried to make it better??? also sorry for terrible indentation)