By selling 4 articles for 1 rupee, a man losses 4%. Had he sold three articles per rupee, the profit would have been = ?

Selling price of 4 article = Rs. 1
Loss = 4%
Cost price of 4 articles
$${\text{ = 1}} \times \frac{{100}}{{96}} = \frac{{100}}{{96}}$$
Cost price of 3 articles
$${\text{ = }}\frac{{100}}{{96}} \times \frac{3}{4} = \frac{{75}}{{96}}$$
New selling price of 3 articles = Rs. 1
$$\eqalign{ & {\text{Profit }}\% \cr & = \frac{{\frac{{1 - 75}}{{96}}}}{{\frac{{75}}{{96}}}} \times 100 \cr & = \frac{{21}}{{75}} \times 100 \cr & = 28\% \cr} $$