The working mother is now a not unusual factor in a child's home life.
I was confused about this sentence because I think "The working mother is NOT an unusual factor in a child's home life." sounds more natural to me. I asked my grammar teacher about it and she said the sentence means that "The mother is a usual factor" rather than "The mother is not a factor." Can't the word "not" put before "a" to negate "unusual"?