twice = two times

Let's rewrite this with "two times" as it may be easier to explain

In the 1980â€™s the rate of increase of the minority population of the United States was nearly

two times what it was in the 1970â€™s

"what it was in" = the rate of increase of

--> The rate of increase of the 1980's was

two times the rate of increase of the 1970's

Let's simplify this to:

X = The rate of increase of the 1980's

Y = The rate of increase of the 1970's

Do we say:

1- X was two times Y

or

2- X was two times of Y?

Since twice is equivalent to "two times", the same logic applies. You should NOT have "of" inserted in between.

_________________

Best Regards,

Paul