I know this sentence is correct:
In the 1980's the rate of increase of the minority population of the United States was nearly twice what it was in the 1970's.
So the part of the sentence I'm concerned with is the twice part. Correct sentence: The rate was twice what it was in the 1970s.
Why do I have to use the "what" here?
Can't I just say: The rate was twice it was in the 1970s.
That sounds incorrect to me, but I don't know why. Thank you for the help!