A shopkeeper allows a discount of 20% on the marked price and still gains 20% on the whole. Find

at what percent above the cost price did he marked his goods.

Let M.P = m     

 discount = 20% 

SP=(100-20)% of MP=80% of m=$\frac{4m}{5}$  $\dots \left(1\right)$           

Let C.P = c                  

gain =20%

SP=(100+20)%of cp=120% of c=$\frac{6c}{5}$$\dots \left(2\right)$

equate (1) and (2)

$\frac{4m}{5}=\frac{6c}{5}$

m:c=3:2

$\frac{3x-2x}{2x}\times 100\%$

$\frac{x}{2x}\times 100\%$=50%

he marked 50% above the  cost price