guaging the difficulty level

Hey guys,
Really strange experience have I had solving this problem under untimed conditions.

"There is a rectangle ABCD with a straight line AE from A to the other opposite edge CD at a point E. The question is: IS that area of the triangle formed by that line ADE > 25?
(1) the length of AD is given as 6
(2) the length of the AE is 10

if I approach this problem from an open mind I will get (B).
The whole AD^2 + DE^2 = 100
so AD^2 + 2*AD*DE + DE^2 = 100 + 2*AD*DE
so (AD + DE)^2 - 100 = 2*AD*DE
so [(AD + DE)^2]/4 - 25 = (1/2)*AD*DE
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I solved this problem without any pressure.

But under timed conditions how the hell am I gonna gauge the difficulty level of this problem? Its not that simple, yet its not that hard. Its just tricky.
My instinct under severe pressure will be to go for (C) because you know two values of the edges of the right triangle and you will get the third and you can find the area.

I saw this problem on manhattan's quant challenge section and I assumed that this wasn't a cakewalk. But I had a biased approached already because of the assumption.
Real GMAT is not as bada$$ as manhattan's challenge so there is a likelihood that I might assume that this is simple and choose C.