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The easiest way will be to equate the volume of the water in the 2 cases.

In the first case volume of water =1/3(pi)r^2h -1/3(pi)r^2.8 (r- radius of cone, h- height of cone)

In the second volume of water = 1/3(pi)r^2h - 1/3(pi)r^2(h-2)

equating the two, and cancelling 8 = h -2, or h=10
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In the second volume of water = 1/3(pi)r^2h - 1/3(pi)r^2(h-2)

can you please explain the second case in details?
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Attachment:
image.jpg

In the figure above, when a conical bottle is rested on its flat base, the water in the bottle is 8 cm from its vertex. When the same conical bottle is turned upside down, the water level is 2 cm from its base. What is the approximate height of the bottle?

A. 5cm
B. 8cm
C. 10cm
D. 12cm
E. 15cm

This is not a GMAT question.

Say the height of the bottle is H and radius is R.
The volume of the water in the two cases is same.

Case 1: Volume of water = Volume of cone - Volume of empty cone on top
Volume of water = \((\frac{1}{3})*\pi*R^2*H - (\frac{1}{3})*\pi*[(8/H)*R]^2*8\)

Case 2: Volume of water = Volume of smaller cone
Volume of water = \((\frac{1}{3})*\pi*[((H-2)/H)R]^2 *(H - 2)\)

\(\frac{1}{3}*\pi*R^2*H - \frac{1}{3}*\pi*[\frac{8}{H}*R]^2*8 = \frac{1}{3}*\pi*[\frac{H-2}{H}*R]^2 *(H - 2)\)

\(R^2*H - [\frac{8}{H}*R]^2*8 = [\frac{H-2}{H}*R]^2 *(H - 2)\)

\(H - \frac{8^3}{H^2} = \frac{(H - 2)^3}{H^2}\)

\(H^3 - 8^3 = (H - 2)^3\)

The value of H for which this would hold would be about 10.22 and hence 10 would be the best answer.
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In either diagram, larger triangle forms a similar triangle with the smaller triangle.

Now, assume height of cone as h. So, height of smaller triangle on left is 8 and on the right is (h-2)

As volumes of the liquid and the cone is same in either diagram, ratio of [volume of liquid : volume of cone] should also be constant in each. Hence, sides will also be in proportion i.e. h/8 = h/(h-2)
Solving, we get h= 0 or 10. As h cannot be 0, Ans is 10 / C
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