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Lets assume,
when the cone is filled till h0, the radius of circular cross section of cone till that height = r0 (which is AB)
when the cone is filled till h1, the radius of circular cross section of cone till that height = r1 (which is CD)
From, similarity of triangels,
AB/CD = OA/OC
=> r0/r1 = h0/h1 ........... (1)
Now,
60% of water was drained
=> initial volume / final volume = 100 / 40
=> [ (1/3) * pi * r0 * r0 * h0] / [ (1/3) * pi * r1 * r1 * h1] = 1 / 0.4
from (1) we can replace the value of r0/r1
=> (h0*h0*h0)/(h1*h1*h1) = 1/0.4
=> h1/h0 = cube root of 0.4 = 0.7368 > 0.4
So, the answer is A.