Hi there,
I have a sentence:
Prices are double what they were two years ago.
I wonder, if there should be a preposition, like "of" or something, follow the word "double"?
Thanks in advance for your help.

Question

Answer

"Of" sounds wrong. You should say "double what they were" or "twice what they were". "Double" and "twice" serve as adjectives. If you wanted to add "of", you would need to insert "the" in order to turn "double" into a noun:
"Prices are the double of what they were two years ago."

Answer

It is correct and clear as written

Answer

Your sentence is correct. The preposition 'of' is not needed.