$\pi (R^2-r^2) = \frac{256}{100}\pi r^2$
$R^2-r^2 = \frac{256}{100} r^2$
$R^2 = \frac{356}{100} r^2$
$R^2 = (\frac{2r\sqrt{89}}{10})^2$
$R^2 = (\frac{36\sqrt{89} }{10})^2$
$R = \frac{36\sqrt{89} }{10}$
ถ้าคำนวณทั่วไป ควรให้เด็กประมาณ $\sqrt{89} \approx 9.43$
จะได้ $R \approx 33.95 \Rightarrow D = 67.9 cm.$
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