Puzzle ZWXM 

Workings

Hint
A little algebra might help.

Answer
¹⁄₈.

Reasoning
If we convert the question to algebra, we have:
((n^2 + 4) x 2 − 3) x 4 − n = 20

Expanding the brackets and simplifying gives:

(2n^2 + 8 − 3) x 4 − n = 20

    (2n^2 + 5) x 4 − n = 20

         8n^2 + 20 − n = 20

              8n^2 − n = 0 (*)

                8n − 1 = 0

                    8n = 1

                     n = ¹⁄₈

In the equation marked (*) zero is also a potential solution, but as the question tells us that we "Start with a number larger than 0" we know that n can't be 0, and therefore we can safely divide by n.

Double-Checking
                 ¹⁄₈
   squared     = ¹⁄₆₄
   add 4       = 4¹⁄₆₄
   double      = 8¹⁄₃₂
   take away 3 = 5¹⁄₃₂
   times 4     = 20¹⁄₈

   subtracting ¹⁄₈ gives 20, as required.