On decreasing the radius of a circle by 30%, its area is decreased by

(d) None of these
Let r be the original radius.
Thus, we have:

Original area $=\pi r^{2}$

Also,

New radius $=70 \%$ of $r$

$=\left(\frac{70}{100} \times r\right)$

$=\frac{7 r}{10}$

New area $=\pi \times\left(\frac{7 r}{10}\right)^{2}$

$=\frac{49 \pi r^{2}}{100}$

Decrease in the area $=\left(\pi r^{2}-\frac{49 \pi r^{2}}{100}\right)$

$=\frac{59 \pi r^{2}}{100}$

Thus, we have:

Decrease in the area $=\left(\frac{59 \pi r^{2}}{100} \times \frac{1}{\pi r^{2}} \times 100\right) \%$

$=51 \%$