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:

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