In purchasing and selling article, a shopkeeper cheats both seller and the buyer by 10%. His percentage gain is

A) 21%

B) 20%

C) 15%

D) 10%

Correct answer:
A) 21%

Description for Correct answer:
10 X 2 = 20
10 X 10 = 1

Totally 21 %


Please provide the error details in above question