Discussion
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: B
Both dimensions shrink, so multiply the two surviving fractions.
\[ \frac{A'}{A} = (1 - 0.20)(1 - 0.10) = 0.80 \times 0.90 = 0.72 \]
\[ \text{decrease} = 1 - 0.72 = 0.28 = 28\% \]
Adding the two losses gives 30%, which overstates the shrinkage by the product \( 0.20 \times 0.10 = 2\% \) - the part counted twice.
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