A towel, when bleached, was found to have lost 20% of its length and 10% of its breadth. The percentage of decrease in area is :

Let the original length = x and original breadth = y
Decrease in area :
$$\eqalign{ & = xy - \left( {\frac{{80}}{{100}}x \times \frac{{90}}{{100}}y} \right) \cr & = \left( {xy - \frac{{18}}{{25}}xy} \right) \cr & = \frac{7}{{25}}xy \cr} $$
∴ Decrease% :
$$\eqalign{ & = \left( {\frac{7}{{25}}xy \times \frac{1}{{xy}} \times 100} \right)\% \cr & = 28\% \cr} $$