Question : A hemispherical bowl of radius 30 cm is filled with water by using a cylindrical glass. If the water from the cylindrical glass is poured 72 times to fill the bowl completely and the height of the glass is 10 cm, then what is the base radius of the cylindrical glass?
Option 1: 7.5 cm
Option 2: 8 cm
Option 3: 5 cm
Option 4: 6 cm
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Correct Answer: 5 cm
Solution :
The volume of a hemisphere $ = \frac{2}{3}\pi r^3$
The volume of a cylinder $ = \pi r^2 h$
Where $r$ is the base radius and $h$ is the height.
Given that the radius of the hemisphere is 30 cm and the height of the cylindrical glass is 10 cm.
Equate the volume of the hemisphere to the volume of the cylindrical glass multiplied by the number of times the glass is poured into the bowl (72 times) to find the base radius of the cylindrical glass.
$⇒\frac{2}{3}\pi (30)^3 = 72 \times \pi r^2 \times 10$
$⇒r = \sqrt{\frac{2 \times (30)^3}{3 \times 72 \times 10}} = 5$
Hence, the correct answer is 5 cm.
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