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A spherical iron ball of radius 10 cm is coated with a layer of ice of uniform thickness that melts at a rate of 50 cm 3 /min. When the thickness of the ice is 5 cm, then the rate at which the thickness (in cm/min) of the ice decreases, is :

1. π 6 5
2. π 9 1
3. π 36 1
4. π 18 1

Correct answer: π 18 1

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