A tradesman sold an article at a loss of 20% If the selling price had been increased by 100 Rs, there would have been a gain of 5%, the cost price of the article is 

Let the C.P at the article be $x$ Rs

S.P at the article at a loss of 20%

$=\frac{80}{100}\times x=\frac{80x}{100}Rs$

S.P at the article at a profit of 5%

$=\frac{105}{100}\times x=\frac{105x}{100}$

According to the question

$\frac{80x}{100}+100=\frac{105x}{100}$

$\frac{105x}{100}-\frac{80x}{100}=100$

$x=\frac{100\times 100}{25}=400Rs$