[ubuntu-mono] [Bug 572607] Re: Incorrect answer in the 'cube' task

[Soren Mogensen]

I actually got this one wrong. Or rather, I think the n^3 answer is
wrong.

The question says "the figure" below, and while it immediately looks
like a cube, there is no way to know whether the figure is hollow. Thus,
the only correct answer would have to be the sum of small cubes visible
from the three sides visible to the user.

Thus for the 6 x 6 x 6 figure, the answer would be 6x6 + 5x6 + 5x5.

Thus, the question should point out that the figure is a complete cube
in order for the n^3 answer to be correct.

-- 
Incorrect answer in the 'cube' task
https://bugs.launchpad.net/bugs/572607
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Status in GBrainy: New
Status in “gbrainy” package in Ubuntu: Confirmed

Bug description:
Binary package hint: gbrainy

The 'correct' answer in the 'Cube' problem is indeed wrong. The task reads:

"How many cubes do you count in the figure below? (not counting the figure)",

and the picture of a big cube subdivided into n^3 small cubes follows (see attachment). The number n is either 4, 5, or 6. GBrainy says that for each n the correct answer is n^3 (i. e. 64 for n = 4, 125 for n = 5 and 216 for n = 6), but obviously that's wrong.

There are also sub-cubes of sizes 2x2x2, 3x3x3 and so on. For example, in the case of n = 4, one can find 99 cubes:

8 = 2^3 cubes of size 3x3x3,
27 = 3^3 cubes of size 2x2x2
and 64 = 4^3 cubes of size 1x1x1.

So, for an arbitrary n the correct answer is 1^3 + 2^3 + ... + n^3. In particular, the answer for n = 5 is 224, the answer for n = 6 is 440, and, as already mentioned, the answer for n =4 is 99.