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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.

Workings

Hints
The first letters of the words are: T, C, C, G, T, B, S, B, D, G.

Answers
----rem = theorem ----oct = concoct ----pso = calypso ----dma = grandma ----edo = torpedo ----rre = bizarre ----sfy = satisfy ----ott = boycott ----hma = drachma ----dpa = grandpa

Workings

Hints
The first letters of the words are: W, O, B, H, D, F, C, N, H, D.

Answers
wat + erf + all = waterfall
ove + rbo + ard = overboard
ban + dst + and = bandstand
hai + rbr + ush = hairbrush
dif + fer + ent = different
fis + her + man = fisherman
con + duc + tor = conductor
new + spa + per = newspaper
hea + dli + ght = headlight
dra + gon + fly = dragonfly

Workings

Hint
The first word ends in k.

Answer
Look before you leap.