Question

if the cp is 90% of sp find the gain % ?

Answer 1

You can answer it like this-
Let the sp be-rupees 100
Since cp=90%ofsp
         cp=90/100*100=90

since cp= rupees 90 and sp is rupees 100
gain=100-90=10 rupees

thus gain%=gain/cp*100
                  =10/90*100
                  =100/9
                  =50/3
                  =16 2/3%
Hopefully that is correct.

Answer 2

According to the question
C=(90/100)S=0.9S

gain%
= ((C-S)/C)  x100
=((0.9S-S)/0.9s) x 100
=(0.8/0.9) x 100
=88.88 %