What is the answer to 90 is 45% of what?

What is the best way to answer questions with a x expression in the question. for example 3x + 10x=?

Which questions would most a reader identify the author's purpose for writing? check all that apply. what type of text is this? what type of reviews did the book receive? how many copies of the book have been sold? what is the central idea? how does this text make one feel?

If u good at math hit me up on insta or sum @basic_jaiden or @ and