GREAT CATCH! You are to be LAUDED!
I used to try to get some conversation going over the incontrovertible FACT that one of the books from the 1940s provided a "can you do the math" discussion which, if followed led directly to 1972 as the answer. No one seemed very interested in this topic.
Right now I can't recall the name of the book or the page this arithmetic exercise was on, but it should be somewhere in the archives here.
I agree that this is an important and noteworthy Watchtower quote.
A-HA! It was the 1943 book "The Truth Shall Set You Free" that had that challenging arithmetic exercise on