Educated guess vs. random guess

Hi folks,

I often read that it's wise to take an educated guess on the next difficult question when you run short on time. However, I asked myself whether this is really the way to go. Consider this:

Optimally, I should solve a problem in 120 seconds. This means that my chance increases from 20% at time zero to 100% after 120s - or by 0,67% per second. Using this rate, you can derive the following table:

Elapsed time | Eliminated questions | Increase of chance | Cumulated chance
0 | 0 | 0 | 20%
8 | 1 | +5% | 25%
17 | 2 | +13% | 33%
45 | 3 | +17% | 50%
120 | 4 | +50% | 100%

Now the issue: If I take a random guess immediately, I will waste ~5 seconds. However, if I read the whole questions and all the answers and then quickly think about which annswers to eliminate, which is neccesary to make an educated guess, I will need ~45 seconds. Let's say I am able to eliminate 2 answers: then, according to the table, I waste 45-17=28 seconds. Maybe I'm lucky to elimante 3 questions, in that case I would neither win nor loose any time. (I won't be able to eliminate >3 answers, otherwise the question can't be considered difficult.)

So, from my point of view to use every second most efficiently it is more reasonable to make a random guess than it is to make an eduacated guess. What do you think?