The adding song cookie monster appeared in with a hippie type
muppet, the hippie did the adding and Cookie Monster did the
subtracting. I saw this segment when I was little and I looked it up
on the Sesame Street Lyrics Archive Website and it said Cookie
Monster was then voiced by Frank Oz but didn't say who the hippie
was voiced by, but my guess would be that the hippie was voiced by
Jerry Nelson.

Andrew