The part is 2x and the percent is 4. You need to find the whole.

The part is 2(10)                    (Substitute x = 10)

Let n represent the whole.

Part = percentwhole

                           (Substitute part = 20 and percent = 0.04)

                   (Divide each side by 0.04)

                                (Cancel common terms)

                                    (Divide: )

20 is 4 of 500.