How to find the volume of a sphere - GRE Quantitative Reasoning

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Question

How much does the volume of a sphere increase if its radius is increased by 50%?

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Answer

Recall the equation for the volume of a sphere:

V = (4/3)πr3

If we increase the radius by 50%, we can represent the new radius as being equal to r + 0.5r = 1.5r.

Replace this into the equation for the volume and simplify:

V2 = (4/3)π(1.5r)3 = (4/3)π(3.375r3)

Rewrite this so that you can compare the two volumes:

V2 = 3.375 * (4/3)πr3 = 3.375 * \[(4/3)πr3\]

This is the same as:

V2 = 3.375 * V

This means that the new volume is 337.5% of the original. However, note that the question asked for the increase, which would be an increase by 237.5%.

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