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 :

Answer: Option D

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