Question
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 :
$$\eqalign{
& = xy - \left( {\frac{{80}}{{100}}x \times \frac{{90}}{{100}}y} \right) \cr
& = \left( {xy - \frac{{18}}{{25}}xy} \right) \cr
& = \frac{7}{{25}}xy \cr} $$
∴ Decrease% :
$$\eqalign{
& = \left( {\frac{7}{{25}}xy \times \frac{1}{{xy}} \times 100} \right)\% \cr
& = 28\% \cr} $$
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