Which expression can be used to find 640% of 700?

Here's a trick. When you see the word "of", replace it with a times symbol. Whenever you see the word "is", replace it with an equals sign:

640% of 700 is 
           ↑         ↑
       × or  ·     =

Now convert 640% to a decimal so it can work in the problem

640% = 6.40

and multiply:

6.40 · 700 =  4480