How much percentage above the cost price should a shopkeeper mark on his goods so that after allowing a discount of 10% he should gain 26% ?

$$\eqalign{ & {\text{CP}} = {\text{100 }} - {\text{ D% }} \cr & \,\,\,\,\,\,\,\,\,\,\, = {\text{100 }} - {\text{ 10}} \cr & \,\,\,\,\,\,\,\,\,\,\, = {\text{90}} \cr & {\text{MP}} = {\text{100 + P% }} \cr & \,\,\,\,\,\,\,\,\,\,\,\,\, = {\text{100 + 26}} \cr & \,\,\,\,\,\,\,\,\,\,\,{\text{ = 126}} \cr & {\text{Difference = 126 }} - {\text{ 90}} \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{\text{ = 36}} \cr & {\text{Required }}\% = \frac{{36}}{{90}} \times 100 = 40\% \cr} $$