If 10% more is gained by selling an article for Rs. 1400 than by selling it for Rs. 1300, the cost of the article is

Let CP = x

$Gain\%=\frac{1300-x}{x}\times 100$

when SP = 1400

$Gain\%=\frac{1400-x}{x}\times 100$

$=\left[\frac{1400-x}{x}\right]\times 100-\left[\frac{1300-x}{x}\right]\times 100$

$10=[1400-x-1300+x]\times \frac{100}{x}\Rightarrow 10=\frac{100\times 100}{x}$

$\Rightarrow \boxed{x=1000}$