Question

cost price of air article is 90% of its selling price find gain%

Answer 1

let the Sp b x
 so the cp=9x/10
profit=sp-cp=x/10
 profit%=$\frac{x}{10} X \frac{10}{9x}X 100 \\ = 9 \frac{1}{11} %$

Answer 2

CP = 90/100 * SP
SP = 100 / 90 * CP

GAIN =  SP - CP =  CP * 100/90  - CP
     = CP [  100/90 - 1 ] =  CP * 10/90

gain % =  [ CP/9 ]  / CP * 100  =  100/9 = 11.11%